Feature request: CURLOPT_POSTFIELDS non-copying wrappers

[Pr0methean]

It would be handy if curl-rust provided a send_with_body wrapper around CURLOPT_POSTFIELDS that took a borrowed byte slice, and a post_fields_move that would take ownership of a boxed slice to send later. That way it would be just as convenient to move the POST body as to copy it, when using this crate with ones like serde_urlencoded that don't integrate with the Form type. (Otherwise I have to add a Cursor<Box<[u8]>> field to my Handler implementation and pass all reads through to it.)