UpdateTask handles PUT requests to /task/{taskID}. The request body must contain a JSON encoded task. Example: req: PUT /task/1 {"ID": 1, "Title": "Learn Go", "Done": true} res: 200 req: PUT /task/2 {"ID": 2, "Title": "Learn Go", "Done": true} res: 400 inconsistent task IDs
(w http.ResponseWriter, r *http.Request)