Fixed RDoc about to_msgpack that actually requires packer
Fixed RDoc about to_msgpack that actually requires packer

IO is still supported for backward compatibility but they actually use
packer. This is because Packer has internal buffer and using IO as the
interface will require Packer to flush the buffer to IO every time.

ojundt/msgpack-ruby,ojundt/msgpack-ruby,ojundt/msgpack-ruby,ojundt/msgpack-ruby,msgpack/msgpack-ruby,cosmo0920/msgpack-ruby,cosmo0920/msgpack-ruby,cosmo0920/msgpack-ruby,msgpack/msgpack-ruby,cosmo0920/msgpack-ruby,msgpack/msgpack-ruby