Fix #837

Unifies how we fetch the client_id from the request and allows us to throw a more appropriate exception when the client_id parameter is missing.

Improves the test method for this validation by checking the culpable method in this particular case. The test was missing this by calling the wrong method.

tests/Grant/AuthCodeGrantTest.php

@@ -335,7 +335,7 @@ class AuthCodeGrantTest extends TestCase
             ]
         );
 
-        $grant->validateAuthorizationRequest($request);
+        $grant->canRespondToAuthorizationRequest($request);
     }
 
     /**