Correctly implement the POST credentials method

https://docs.openstack.org/api-ref/identity/v3/index.html?expanded=authenticating-with-an-application-credential-detail,create-credential-detail#credentials
specifies that the credentials endpoint expects a credential object and returns one. This patch ensures the data is properly encoded and decoded.

Author: Sven Wiltink

src/Identity/v3/Api.php

@@ -691,6 +691,7 @@ public function postCredentials(): array
         return [
             'method' => 'POST',
             'path'   => 'credentials',
+            'jsonKey' => 'credential',
             'params' => [
                 'blob'      => $this->params->blob(),
                 'projectId' => $this->params->projectId(),

src/Identity/v3/Models/Credential.php

@@ -39,6 +39,8 @@ class Credential extends OperatorResource implements Creatable, Updateable, Retr
         'user_id'    => 'userId',
     ];
 
+    protected $resourceKey = 'credential';
+
     /**
      * {@inheritdoc}
      */