Avoid decoding msgpack request body as UTF-8

Currently, if a request body with a Content-Type of "application/x-msgpack" is sent into Puppet Server, it will inappropriately be decoded into a string as UTF-8. Instead, like is done for application/octet-stream, the raw bytes of the request body should be passed on through the indirector in Ruby Puppet, without any preliminary decoding.

The production code change for this ticket may just involve changing this line to:

(nil "" "application/octet-stream" "application/x-msgpack") body

Along with this change, some sort of automated "integration" test should be written to validate that the msgpack payload is preserved / handled correctly through the request pipeline.