Fix Protobuf support - HTTP headers already written

Prior to this commit, the `ProtobufHttpMessageConverter` would call
`outputMessage.getBody()` at the beginning of the `writeInternal`
method, thus writing HTTP headers. Since this method is trying to write
"x-protobuf" headers after that, protobuf support wasn't working
properly for the default "x-protobuf" media type.

Thanks Toshiaki Maki for the repro project!

Also fixed:
* improve `MockHttpOutputMessage` behavior to reproduce the read-only
state of HTTP headers once they've been written.

Issue: SPR-12229