title: Google OAuth 2 and state parameter values need to be registered in redirect url
body: <p>The state parameter according to the Google Oauth 2.0 docs:</p>
  
  <blockquote>
    <p>Indicates any state which may be useful to your application upon receipt of the response. The Google Authorization Server roundtrips this parameter, so your application receives the same value it sent. Possible uses include redirecting the user to the correct resource in your site, nonces, and cross-site-request-forgery mitigations.</p>
  </blockquote>
  
  <p>I'd like to use the state parameter as a means to know which subdomain the original oauth request was initiated from.  But the redirect_state parameter appears to need to be registered as part of one of the "Authorized Redirect URIs".  If not, I get:</p>
  
  <blockquote>
    <p>Error: redirect_uri_mismatch
    The redirect URI in the request: <a href="http://my_server.com/complete/google-oauth2/?redirect_state=2" rel="noreferrer">http://my_server.com/complete/google-oauth2/?redirect_state=2</a> did not match a registered redirect URI</p>
  </blockquote>
  
  <p>I would like a solution that does not require registering every possible redirect_state value in the authorized redirect URIs as that isn't very maintainable.  Ideas?</p>