Unwanted double quotes around server response in Python Flask-RESTful

Question

I have an issue using python Flask-RESTful.
Consider the code below:

Class ServiceAPI(Resource):
    def post(self):
        return 'Hello, World!', 200

Then the client receives:

"Hello, World!"    

instead of

Hello, World!

the extra double quotes are undesired. How can I avoid them? I don't want to solve the issue by stripping/trimming the data received, in the client.