fix: change mr.merge() to use 'post_data'

MR #1121 changed
mr.merge() to use 'query_data'. This appears to have been wrong.

From the Gitlab docs they state it should be sent in a payload body
https://docs.gitlab.com/ee/api/README.html#request-payload since
mr.merge() is a PUT request.

  > Request Payload

  > API Requests can use parameters sent as query strings or as a
  > payload body. GET requests usually send a query string, while PUT
  > or POST requests usually send the payload body

Fixes: #1452
Related to: #1120

gitlab/v4/objects/merge_requests.py

@@ -342,7 +342,7 @@ def merge(
         if merge_when_pipeline_succeeds:
             data["merge_when_pipeline_succeeds"] = True
 
-        server_data = self.manager.gitlab.http_put(path, query_data=data, **kwargs)
+        server_data = self.manager.gitlab.http_put(path, post_data=data, **kwargs)
         self._update_attrs(server_data)