text: Title: unicode issues
  Body: When I follow the example to retrieve data I'm greeted with the following stacktrace:
  
  ```
  In [4]: supabase.table("countries").select("*").execute()
  ---------------------------------------------------------------------------
  UnicodeEncodeError                        Traceback (most recent call last)
  <ipython-input-4-91499f52c962> in <module>
  ----> 1 supabase.table("countries").select("*").execute()
  
  /usr/lib/python3.8/site-packages/supabase_py/client.py in table(self, table_name)
       72         Alternatively you can use the `._from()` method.
       73         """
  ---> 74         return self.from_(table_name)
       75 
       76     def from_(self, table_name: str) -> SupabaseQueryBuilder:
  
  /usr/lib/python3.8/site-packages/supabase_py/client.py in from_(self, table_name)
       79         See the `table` method.
       80         """
  ---> 81         query_builder = SupabaseQueryBuilder(
       82             url=f"{self.rest_url}/{table_name}",
       83             headers=self._get_auth_headers(),
  
  /usr/lib/python3.8/site-packages/supabase_py/lib/query_builder.py in __init__(self, url, headers, schema, realtime, table)
       71             **headers,
       72         }
  ---> 73         self.session = AsyncClient(base_url=url, headers=headers)
       74         # self._subscription = SupabaseRealtimeClient(realtime, schema, table)
       75         # self._realtime = realtime
  
  /usr/lib/python3.8/site-packages/httpx/_client.py in __init__(self, auth, params, headers, cookies, verify, cert, http2, proxies, timeout, limits, pool_limits, max_redirects, event_hooks, base_url, transport, app, trust_env)
     1209         trust_env: bool = True,
     1210     ):
  -> 1211         super().__init__(
     1212             auth=auth,
     1213             params=params,
  
  /usr/lib/python3.8/site-packages/httpx/_client.py in __init__(self, auth, params, headers, cookies, timeout, max_redirects, event_hooks, base_url, trust_env)
       98         self._auth = self._build_auth(auth)
       99         self._params = QueryParams(params)
  --> 100         self.headers = Headers(headers)
      101         self._cookies = Cookies(cookies)
      102         self._timeout = Timeout(timeout)
  
  /usr/lib/python3.8/site-packages/httpx/_models.py in __init__(self, headers, encoding)
      549             self._list = list(headers._list)
      550         elif isinstance(headers, dict):
  --> 551             self._list = [
      552                 (
      553                     normalize_header_key(k, lower=False, encoding=encoding),
  
  /usr/lib/python3.8/site-packages/httpx/_models.py in <listcomp>(.0)
      553                     normalize_header_key(k, lower=False, encoding=encoding),
      554                     normalize_header_key(k, lower=True, encoding=encoding),
  --> 555                     normalize_header_value(v, encoding),
      556                 )
      557                 for k, v in headers.items()
  
  /usr/lib/python3.8/site-packages/httpx/_utils.py in normalize_header_value(value, encoding)
       54     if isinstance(value, bytes):
       55         return value
  ---> 56     return value.encode(encoding or "ascii")
       57 
       58 
  
  UnicodeEncodeError: 'ascii' codec can't encode character '\u2026' in position 50: ordinal not in range(128)
  
  In [5]: data = supabase.table("countries").select("*").execute()
  ---------------------------------------------------------------------------
  UnicodeEncodeError                        Traceback (most recent call last)
  <ipython-input-5-a2ce57b52ae2> in <module>
  ----> 1 data = supabase.table("countries").select("*").execute()
  
  /usr/lib/python3.8/site-packages/supabase_py/client.py in table(self, table_name)
       72         Alternatively you can use the `._from()` method.
       73         """
  ---> 74         return self.from_(table_name)
       75 
       76     def from_(self, table_name: str) -> SupabaseQueryBuilder:
  
  /usr/lib/python3.8/site-packages/supabase_py/client.py in from_(self, table_name)
       79         See the `table` method.
       80         """
  ---> 81         query_builder = SupabaseQueryBuilder(
       82             url=f"{self.rest_url}/{table_name}",
       83             headers=self._get_auth_headers(),
  
  /usr/lib/python3.8/site-packages/supabase_py/lib/query_builder.py in __init__(self, url, headers, schema, realtime, table)
       71             **headers,
       72         }
  ---> 73         self.session = AsyncClient(base_url=url, headers=headers)
       74         # self._subscription = SupabaseRealtimeClient(realtime, schema, table)
       75         # self._realtime = realtime
  
  /usr/lib/python3.8/site-packages/httpx/_client.py in __init__(self, auth, params, headers, cookies, verify, cert, http2, proxies, timeout, limits, pool_limits, max_redirects, event_hooks, base_url, transport, app, trust_env)
     1209         trust_env: bool = True,
     1210     ):
  -> 1211         super().__init__(
     1212             auth=auth,
     1213             params=params,
  
  /usr/lib/python3.8/site-packages/httpx/_client.py in __init__(self, auth, params, headers, cookies, timeout, max_redirects, event_hooks, base_url, trust_env)
       98         self._auth = self._build_auth(auth)
       99         self._params = QueryParams(params)
  --> 100         self.headers = Headers(headers)
      101         self._cookies = Cookies(cookies)
      102         self._timeout = Timeout(timeout)
  
  /usr/lib/python3.8/site-packages/httpx/_models.py in __init__(self, headers, encoding)
      549             self._list = list(headers._list)
      550         elif isinstance(headers, dict):
  --> 551             self._list = [
      552                 (
      553                     normalize_header_key(k, lower=False, encoding=encoding),
  
  /usr/lib/python3.8/site-packages/httpx/_models.py in <listcomp>(.0)
      553                     normalize_header_key(k, lower=False, encoding=encoding),
      554                     normalize_header_key(k, lower=True, encoding=encoding),
  --> 555                     normalize_header_value(v, encoding),
      556                 )
      557                 for k, v in headers.items()
  
  /usr/lib/python3.8/site-packages/httpx/_utils.py in normalize_header_value(value, encoding)
       54     if isinstance(value, bytes):
       55         return value
  ---> 56     return value.encode(encoding or "ascii")
       57 
       58 
  
  UnicodeEncodeError: 'ascii' codec can't encode character '\u2026' in position 50: ordinal not in range(128)
  ```
  
  I've tried this on Python 3.7, 3.8, and 3.9 all with similar results. I've also tried different OSes (OSX, Linux), but both fail in similar fashion.