More client credentials test

tests/api/ClientCredentialsMissingClientSecretCept.php

@@ -0,0 +1,16 @@
+<?php
+$I = new ApiTester($scenario);
+$I->wantTo('get an access token using the client credentials grant, missing client secret');
+$I->sendPOST(
+    'client_credentials.php/access_token',
+    [
+        'grant_type' => 'client_credentials',
+        'client_id'  => 'myawesomeapp'
+    ]
+);
+$I->canSeeResponseCodeIs(400);
+$I->canSeeResponseIsJson();
+$I->seeResponseContainsJson([
+    'error'   => 'invalid_request',
+    'message' => 'The request is missing a required parameter, includes an invalid parameter value, includes a parameter more than once, or is otherwise malformed. Check the "client_secret" parameter.'
+]);