Fix sending post requests

During the refactor of get and post methods this bug was introduced, when setting the body, that method
returns an modified object with the supplied body (immutable).

src/Teamleader.php

@@ -186,7 +186,7 @@ private function doRequest(string $method, string $uri, array $parameters = []):
             }
 
             $body = $this->streamFactory->createStream($encodedParameters);
-            $request->withBody($body);
+            $request = $request->withBody($body);
         }
 
         $response = $this->client->sendRequest($request);