Automatic merge from submit-queue

Don't double encode protobuf runtime.Unknown

When using runtime.Object and runtime.RawExtension, passing
runtime.Unknown to protobuf serializer should result in the raw message
being serialized (since all normal protobuf objects are
runtime.Unknown).

Also, avoid setting a content-type when decoding a protobuf object
except when the content appears to be proto.

@wojtek-t