Support payloads when using query parameters for POSTs/PUTs

Previously, query string parameters were always inserted into the payload for POSTs and PUTs, overwriting anything in the payload.

When a payload is not included with the request, the query string parameters are still inserted into the payload.

Breaking change:
Query string parameters are no longer inserted into the payload when a payload has already been included. The payload remains as it is, and the query string parameters remain in the URL.