- When the `Body` is created from a buffer of bytes (such as
`Body::from("hello")`), we can skip some bookkeeping that is
normally required for streaming bodies.
- Orthogonally, optimize encoding body chunks when the strategy
is to flatten into the headers buf, by skipping the EncodedBuf
enum.
- Downgrades internal semantics to HTTP/1.0 if peer sends a message with
1.0 version.
- If downgraded, chunked writers become EOF writers, with the connection
closing once the writing is complete.
- When downgraded, if keep-alive was wanted, the `Connection: keep-alive`
header is added.
Closes#1304
