Merge pull request #53 from mollie/issue-51/fix-send-request

Only json_encode data when there is data. Fixes #51

src/Message/Request/AbstractMollieRequest.php

@@ -72,7 +72,7 @@ protected function sendRequest($method, $endpoint, array $data = null)
             [
                 'Authorization' => 'Bearer ' . $this->getApiKey()
             ],
-            json_encode($data)
+            ($data === null) ? null : json_encode($data)
         );
 
         return json_decode($response->getBody(), true);