title: Identity Server 4 GetSchemeSupportsSignOutAsync returns incorrect response
body: <p>I've setup an open id connect provider, Google in this case, using the AddOpenIdConnect extension method in dotnet core. From the discovery document: </p>
  
  <pre><code>https://accounts.google.com/.well-known/openid-configuration 
  </code></pre>
  
  <p>it does not seem that google supports federated sign-out because there is no end_session endpoint. However, in Identity Server 4, the call:</p>
  
  <pre><code>var providerSupportsSignout = await HttpContext.GetSchemeSupportsSignOutAsync(idp);
  </code></pre>
  
  <p>returns true. So during Logout it tries to sign out of google using:</p>
  
  <pre><code>return SignOut(new AuthenticationProperties { RedirectUri = url }, vm.ExternalAuthenticationScheme);
  </code></pre>
  
  <p>which throws an exception:</p>
  
  <pre><code>InvalidOperationException: Cannot redirect to the end session endpoint, the configuration may be missing or invalid.
  </code></pre>
  
  <p>Is this a bug in Identity Server 4 or is there a configuration property that needs to be set when setting up the Oidc provider so that this extension method will pickup that the provider does not support signout?</p>