Remove Body's `Once` variant

[seanmonstar]

[seanmonstar]

It was noted that the FFI body type makes use of Body::empty(). I think there's 2 ways we can fix that:

1. Add a pub(crate) fn ffi() -> Body that just starts the type like this: https://github.com/hyperium/hyper/blob/509672aada0af68a91d963e69828c6e31c44cb7b/src/body/body.rs#L240
2. Or, keep an Empty variant. I don't think I like this option as much, but welcome feedback.

[seanmonstar]

cc @Xuanwo

[Xuanwo]

1. Add a pub(crate) fn ffi() -> Body that just starts the type like this

Let me give this a try!