'Refreshes the access_token.
Since the underlying App Engine app_identity implementation does its own
caching we can skip all the storage hoops and just to a refresh using the
API.
Args:
http_request: callable, a callable that matches the method signature of
httplib2.Http.request, used to make the refresh request.
Raises:
AccessTokenRefreshError: When the refresh fails.'

def _refresh(self, http_request):

    try:
        (token, _) = app_identity.get_access_token(self.scope)
    except app_identity.Error as e:
        raise AccessTokenRefreshError(str(e))
    self.access_token = token
