Feature request: CURLOPT_POSTFIELDS non-copying wrappers

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>` field to my Handler implementation and pass all reads through to it.)