rename post_body_formatter to request_body_formatter

[galiat]

Just a thought, but in the config, post_body_formatter also formats the body for put & patch so maybe it should be re-named. I'd be happy to rename it to request_body_formatter with backwards compatibility to support those who currently use post_body_formatter.

[oestrich]

I think this makes sense. 👍

[oestrich]

Closing because of #244