Add ability to set Content-Type,Accept headers in HTTP client

Right now the THttpTransport client does not specify a Content-Type header when connecting to an HTTP server, nor does it allow for resetting the Accept header. Neither headers can be set using the current CustomHeader properties. If these headers could somehow be set based on the protocol being used (like in the JavaScript library) then the server instance could use it to properly infer what the inbound and outbound protocols should be.