Fix unit tests.

Update unit tests to support RFC6750 compatibility change made in version 1.9.0 of the bshaffer oauth2 server library.

tests/AuthorizationTest.php

@@ -123,7 +123,7 @@ public function invokeExpiredToken()
 
         $this->assertSame(401, $response->getStatusCode());
         $this->assertSame(
-            '{"error":"expired_token","error_description":"The access token provided has expired"}',
+            '{"error":"invalid_token","error_description":"The access token provided has expired"}',
             (string)$response->getBody()
         );
     }