title: username is null in DotNetOpenAuth2.ResourceServer.VerifyAccess
body: <p>I have implemented an authorization server based on the sample and am receiving an access token in response to client credentials request. From my understanding this access token has a null username because it is not tied to a user.</p>
  
  <p>I have implemented a resource server also based on the sample. When I try to validate the access token in my wcf server (resource server) in OAuthAuthorizationManager.VerifyOAuth2 I get an ArgumentNullException for username from</p>
  
  <pre><code>var error = resourceServer.VerifyAccess(httpRequestInfo, out result);
  </code></pre>
  
  <p>How can I modify OAuthAuthorizationManager to allow a null username?</p>
  
  <p>Do I create a generic principal on the fly and assign it to the scope in the token.
  i.e. should I use </p>
  
  <pre><code>var error = resourceServer.VerifyAccess(httpRequestInfo, out userName, out scope);
  </code></pre>
  
  <p>instead?</p>