Adding a new internal API nn_msg_replace_body to replace the body on an existing message leaving the header intact.

This enhances the ability to create protocols that substantially preprocess or rewrite the underlying message during the send/receive process.

Signed-off-by: Drew Crawford <drew@sealedabstract.com>
Merge pull request #256 from drewcrawford/rewrite-msg

nn_msg_replace_body