client_id from session not used in fetch_token

[jvanasco]

I got tripped up with the example doc on using fetch_token from the overview

https://requests-oauthlib.readthedocs.io/en/latest/

github = OAuth2Session(client_id, state=session['oauth_state'])
token = github.fetch_token(token_url, client_secret=client_secret,
                           authorization_response=request.url)

This doesn't seem to work against custom servers that require a client_id. The logic in fetch_token doesn't pull the client_id off the session, and consequently it's not sent upstream:

https://github.com/requests/requests-oauthlib/blob/master/requests_oauthlib/oauth2_session.py#L200

it looks like we can just add 2 lines as a fallback condition to pull the client_id out:

     client_id = kwargs.get('client_id', '')
+    if not client_id and isinstance(self._client, WebApplicationClient):
+        client_id = self._client.client_id

[jvanasco]

This is tied to a regression in oauthlib. I have a PR there and will create a new ticket or reopen this one when that is merged and I can write a PR for this library.