Make get_request_data take into account the content-type header
    
    If the user has specified a content-type, we can assume that they really
    wanted to pass us a JSON blob and thus we can force the
    flask.request.get_json() method.
    
    This reduces the code duplication
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>