Don't convert incoming binary payloads to String.

Hi JeanFrancois

The ```StreamTransport.onBodyPartReceived()``` method converts any incoming payload to a String, irrespective of Content-Type.

My pull-request is just a proposal, it has "magic numbers": Content-Type and application/octet-stream. Those should be refering to public constants somewhere in the code base, I guess. And furthermore, you might want to make it more flexible on what content-types a client considers being binary.

Cheers, Christian.