Two patches to make the client work better with a picky resource

[ferrix]

I have a picky backend that has two features not supported by the Client class.

a) It wants JSON in the POST body instead of x-www-form-encoded, so I peek into the headers and don't overwrite Content-Type if it is already set, default behavior remains b) cares about the Authentication realm bit, I added a parameter that sets the realm in the request, default behavior remains.