Add support for media types needed for streaming request/responses.

As per the design document created as part of MESOS-3601, we need to add support for the additional media types proposed to our API handlers for supporting request streaming. These headers would also be used by the server in the future for streaming responses.

The following media types needed to be added:

Message-Accept: Enables the client to perform content negotiation for the contents of the stream. The supported values for this header would be application/json and application/x-protobuf.
Message-Content-Type: The content type of the RecordIO stream sent by the server. The supported values for this header would be application/json and application/x-protobuf.

The Content-Type for the response would be application/recordio. For more details/examples see the alternate proposal section of the design doc:

https://docs.google.com/document/d/1OV1D5uUmWNvTaX3qEO9fZGo4FRlCSqrx0IHq5GuLAk8/edit#