text: Title: AttributeError: 'asyncpg.pgproto.pgproto.UUID' object has no attribute 'replace'
  Body: * GINO version: 1.0.1
  * Python version: 3.8.2
  * asyncpg version: 0.20.1
  * PostgreSQL version: 12.3 (Ubuntu 12.3-1.pgdg20.04+1)
  
  ### Description
  
  I'm trying to use UUID value as unique Id in my model
  
  
  ```
  from . import db
  from uuid import uuid4
  from sqlalchemy.dialects.postgresql import UUID
  
  
  class User(db.Model):
      __tablename__ = "users"
  
      id = db.Column(UUID(as_uuid=True), primary_key=True, unique=True, index=True, nullable=False, default=uuid4)
      login = db.Column(db.String(255), nullable=False, unique=True)
      password = db.Column(db.String(255), nullable=True)
      full_name = db.Column(db.String(255))
      last_login = db.Column(db.DateTime, nullable=True)
      is_superuser = db.Column(db.Boolean, nullable=False, default=False)
      is_staff = db.Column(db.Boolean, nullable=False, default=True)
      remark = db.Column(db.String)
  ```
  
  My controller is 
  
  ```
  class UserModel(BaseModel):
      login: str
      password: str
      full_name: str
      is_superuser: bool = False
      is_staff: bool = True
      remark: str = None
  
  
  @router.post("/users")
  async def add_user(user: UserModel):
  
      rv = await User.create(login=user.login,
                             password=user.password,
                             full_name=user.full_name,
                             is_superuser=user.is_superuser,
                             is_staff=user.is_staff,
                             remark=user.remark
                             )
      return rv.to_dict()
  ```
  
  ### What I Did
  
  When I'm trying to post a new user to db via swagger UI I got this error:
  
  ```
  INFO:     127.0.0.1:38548 - "POST /users HTTP/1.1" 500 Internal Server Error
  ERROR:    Exception in ASGI application
  Traceback (most recent call last):
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/uvicorn/protocols/http/httptools_impl.py", line 386, in run_asgi
      result = await app(self.scope, self.receive, self.send)
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/uvicorn/middleware/proxy_headers.py", line 45, in __call__
      return await self.app(scope, receive, send)
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/fastapi/applications.py", line 181, in __call__
      await super().__call__(scope, receive, send)
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/starlette/applications.py", line 111, in __call__
      await self.middleware_stack(scope, receive, send)
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/starlette/middleware/errors.py", line 181, in __call__
      raise exc from None
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/starlette/middleware/errors.py", line 159, in __call__
      await self.app(scope, receive, _send)
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/gino_starlette.py", line 79, in __call__
      await self.app(scope, receive, send)
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/starlette/exceptions.py", line 82, in __call__
      raise exc from None
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/starlette/exceptions.py", line 71, in __call__
      await self.app(scope, receive, sender)
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/starlette/routing.py", line 566, in __call__
      await route.handle(scope, receive, send)
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/starlette/routing.py", line 227, in handle
      await self.app(scope, receive, send)
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/starlette/routing.py", line 41, in app
      response = await func(request)
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/fastapi/routing.py", line 196, in app
      raw_response = await run_endpoint_function(
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/fastapi/routing.py", line 147, in run_endpoint_function
      return await dependant.call(**values)
    File "./src/crm/views/users.py", line 30, in add_user
      rv = await User.create(login=user.login,
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/gino/crud.py", line 444, in _create_without_instance
      return await cls(**values)._create(bind=bind, timeout=timeout)
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/gino/crud.py", line 478, in _create
      for k, v in row.items():
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/sqlalchemy/engine/result.py", line 207, in items
      return [(key, self[key]) for key in self.keys()]
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/sqlalchemy/engine/result.py", line 207, in <listcomp>
      return [(key, self[key]) for key in self.keys()]
    File "/home/petr/crm/.venv/lib/python3.8/site-packages/sqlalchemy/dialects/postgresql/base.py", line 1328, in process
      value = _python_UUID(value)
    File "/usr/lib/python3.8/uuid.py", line 166, in __init__
      hex = hex.replace('urn:', '').replace('uuid:', '')
  AttributeError: 'asyncpg.pgproto.pgproto.UUID' object has no attribute 'replace'
  ```
  