title: finishedWithAuth method not gets called after authentication completion
body: <p>I am doing Google+ Sign-In for iOS.</p>
  
  <p>I am calling authentication method like</p>
  
  <pre><code>  [signIn authenticate];
  </code></pre>
  
  <p>I am not using signin butting.</p>
  
  <p>In this case the following method not gets called after authentication</p>
  
  <pre><code>- (void)finishedWithAuth: (GTMOAuth2Authentication *)auth
                     error: (NSError *) error
  </code></pre>
  
  <p>Please let me know is there any way to handle the callback?</p>