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:

   hyper/src/body/body.rs

   Line 240 in 509672a

   self.kind = Kind::Ffi(crate::ffi::UserBody::new());

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!