make GlpiService.request use json parameter

so it would be possible to make "low-level" requests. for instance, [assign users to ticket](https://forum.glpi-project.org/viewtopic.php?pid=340693#p340693) is impossible, if you use `data` parameter instead of `json`, because `PreparedRequest.prepare_body` messes up arrays.

glpi/glpi.py

@@ -324,7 +324,7 @@ def request(self, method, url, accept_json=False, headers={},
         try:
             response = requests.request(method=method, url=full_url,
                                         headers=headers, params=params,
-                                        data=data, **kwargs)
+                                        data=data, json=json,**kwargs)
         except Exception:
             logger.error("ERROR requesting uri(%s) payload(%s)" % (url, data))
             raise