text: Title: falcon.uri.encode_value does not encode the percent character
  Body: [`falcon.uri.encode_value`](https://falcon.readthedocs.io/en/latest/api/util.html#falcon.uri.encode_value) says:
  > An escaped version of *uri*, where all disallowed characters have been percent-encoded.
  
  However, it seems that the percent character itself is not encoded...
  
  Cf
  ```python
  >>> from urllib.parse import quote
  >>> from falcon import uri
  >>> quote('%26')
  '%2526'
  >>> uri.encode('%26')
  '%26'
  >>> uri.encode_value('%26')
  '%26'
  >>> uri.decode(uri.encode_value('%26'))
  '&'
  >>> uri.decode(quote('%26'))
  '%26'
  ```