Fix the bug that the return of request.get_data() is not string.

[tp6vup54]

I just find that in the flask webhook example, request.get_data() return a value whose type is 'byte' instead of 'string'. And it will cause an error shown below

ValueError: json_type should be a json dict or string.

This commit is a patch that decode the byte value into string using decode('utf-8').