Update connection.py

Fixed two issues:
1. When access_token expired, the return code is "requests.codes.BAD_REQUEST", so removed this from "if statement" so that client can call _refresh;
2. In _refresh request, the payload should be '{"grant_type":"refresh_token","refresh_token":self.refresh_token}', not "refreshToken".
Merge pull request #1 from autumnw/master

Update connection.py