Handling ProtoStream and Protobuf interoperability

This issue concerns non-Java clients using Google Protobuf libraries with the x-protobuf encoding.

If you set the media type in the client to application/x-protobuf, the server won't be able to transcode it to application/x-protostream because there's no reference to application/x-protobuf in ProtostreamTranscoder.

3 questions that we need to answer clearly in the docs:

1. How does a user avoid using WrappedMessage?
2. Assuming the wire format is compatible, what server features work when the value is not wrapped in a WrappedMessage?
3. If some important server features need the value to be wrapped in WrappedMessage, how can a client ask the server to transcode an unwrapped value to a wrapped value?