bugfix(headers): default headers should overwrite built in headers

src/Client/ClientImplementation.php

@@ -151,7 +151,7 @@ class ClientImplementation implements Client, LoggerAwareInterface
             'Accept-Encoding' => '*',
             'Content-Type' => $this->serializer->getMime()
         );
-        $headers = array_merge($this->defaultRequestHeaders, $headers);
+        $headers = array_merge($headers, $this->defaultRequestHeaders);
         return array_merge($headers, $request->getHeaders());
     }