Add #to_msgpack to nil, true, false, String, etc.
Add #to_msgpack to nil, true, false, String, etc.

Thanks to @taichi for a prototype implementation.

kou/msgpack-ruby,nurse/msgpack-ruby,kou/msgpack-ruby,larskanis/msgpack-ruby,kou/msgpack-ruby,ojundt/msgpack-ruby,nurse/msgpack-ruby,cosmo0920/msgpack-ruby,okkez/msgpack-ruby,larskanis/msgpack-ruby,cosmo0920/msgpack-ruby,nurse/msgpack-ruby,ojundt/msgpack-ruby,nurse/msgpack-ruby,okkez/msgpack-ruby,ojundt/msgpack-ruby,okkez/msgpack-ruby,larskanis/msgpack-ruby,msgpack/msgpack-ruby,cosmo0920/msgpack-ruby,cosmo0920/msgpack-ruby,msgpack/msgpack-ruby,larskanis/msgpack-ruby,okkez/msgpack-ruby,msgpack/msgpack-ruby,ojundt/msgpack-ruby,kou/msgpack-ruby