Getting refresh_token from existing session in client credentials refresh

[wrongu]

Fixes #10

I had the same issue with refreshing tokens described in #10. The trouble is that the Oauth2Session.refresh_token function looks for a refresh_token key in the session's existing token (see here).

[wrongu]

The tests fail for me for different reasons locally and on Travis.. and none of them are things that should be affected by the change.

• Locally, the casette for document views isn't returning 'Biological Sciences' as a key in doc.reader_count_by_subdiscipline. Instead, it has a 'Agricultural and Biological Sciences' entry..
• On Travis, no access_token is being returned by the mock auth requests. My guess is that this is because the host expected by get_authenticated_session.yaml has port 443 but the Mendeley object in the tests has the default host. This is just a guess...

Any ideas what I need to do to get the tests working?