code: def prepare(self):
          
          super(RequestHandler, self).prepare()
          if self.request.headers.get('content-type', '').startswith(self.JSON):
              self.request.body = escape.json_decode(self.request.body)
docs: Prepare the incoming request, checking to see the request is sending
          JSON content in the request body. If so, the content is decoded and
          assigned to the json_arguments attribute.