Fix OAuth Authorization

This edit fix the oAuth issue that raised when changed the POST to use  json_encode instead of http_build_query mautic#216

lib/Auth/OAuth.php

@@ -592,7 +592,7 @@ protected function prepareRequest($url, array $headers, array $parameters, $meth
             //OAuth 2.0
             $this->log('making request using OAuth2 spec');
 
-            $parameters['access_token'] = $this->_access_token;
+            $headers[] = 'Authorization: Bearer ' . $this->_access_token;
         }
 
         return [$headers, $parameters];