[ENH] Change Content-Type Header to application/json when calling Service

[Siminho]

I wanted to create an external Endpoint of a Servcie which is called by the service manager. Then I realized, the service manager always sends the corresponding data (msg, botName, channel, ...) as a json string in the body, but with the Content-Type Header set to text/html. This leads to a problem, that I have to define my Endpoint in way, that I expect a String, which is then transformed into json and then eventually transformed into an object. It could be much easier, if the Content-Type is set to application/json and the Endpoint directly expects an object which can be derived from the json.
An Endpoint could look like this: