Fixed authentication using Dropbox

Fixed authentication using Dropbox as reported in https://github.com/gae-init/gae-init-auth/issues/34 based on example code provided at https://github.com/lepture/flask-oauthlib/blob/master/example/dropbox.py
```access_token_params={'grant_type': 'authorization_code'}```
just remove this line (for each provider - microsoft, dropbox, instagram, linked in, reddit) because oauthlib adds it automatically in [web_application.py](https://github.com/idan/oauthlib/blob/master/oauthlib/oauth2/rfc6749/clients/web_application.py#L126)

P.S. but reddit provider still be broken