Use get_json instead of json

The `request.json` method now calls `get_json`, which raises `BadRequest` if there is no json data or some error with it. So, it cannot be directly used as a check for presence of json data. This code currently returns a bad request if content type is `application/json` but json data is empty.

https://github.com/mitsuhiko/flask/blob/master/flask/wrappers.py#L110