Adding support for JSON strings into the request body.

Hello there, 

I was trying to send data to LinkedIn API with a POST request, but came accross a particular problem. Whenever I was sending data to the API, the Stream Client was changing it into a http querry...

So i added support for JSON by adding a Content-type header with the value of application/json. 

To me it makes sens that if that is the header of the request, we expect a valid JSON string to be passed and not an http querry.

Let me know what you guys think.