+++++++++++++++++++++++++
- Add compliance fix for Slack.
- Add compliance fix for Mailchimp.
- ``TokenRequestDenied`` exceptions now carry the entire response, not just the
status code.
- Pass through keyword arguments when refreshing tokens automatically.
- Send authorization in headers, not just body, to maximize compatibility.
- More getters/setters available for OAuth2 session client values.
- Allow sending custom headers when refreshing tokens, and set some defaults.