title: How do you reject a Katana Bearer token's identity
body: <p>How can I reject An identity? My class inherits from OAuthBearerAuthenticationProvider and I have an override of ValidateIdentity?</p>
  
  <p>I have tried setting context.Rejected(); or context.SetError();  and throwing an exception but my controllers still get called.  OAuthBearerAuthenticationHandler does call my class so I know I have the setup correct.</p>
  
  <p>my current failing code</p>
  
  <pre><code>        public void ConfigureAuth ( IAppBuilder app )
          {
              // Enable the application to use a cookie to store information for the signed in user
              // and to use a cookie to temporarily store information about a user logging in with a third party login provider
              app.UseCookieAuthentication(new CookieAuthenticationOptions());
              app.UseExternalSignInCookie(DefaultAuthenticationTypes.ExternalCookie);
  
              // Enable the application to use bearer tokens to authenticate users
              app.UseOAuthBearerAuthentication ( new OAuthBearerAuthenticationOptions ()
              {
                  Provider = new OAuthBearerAuthenticationProvider ()
                  {
                      OnValidateIdentity = async ctx =&gt; { ctx.Rejected (); }
                  }
              } );
              app.UseOAuthBearerTokens(OAuthOptions);
  }
  </code></pre>