Added because the Dropbox OAuth2 flow doesn't
        work when scope is passed in, which is empty.

def _strip_empty_keys(self, params):
        """Added because the Dropbox OAuth2 flow doesn't
        work when scope is passed in, which is empty.
        """
        keys = [k for k, v in params.items() if v == '']
        for key in keys:
            del params[key]