fix: No more BytesWarnings

Fixes #1236.

This patch makes all header operations operate on `bytes` and converts all headers and values to bytes before operation. With a follow up patch to `hpack` it should also increase efficiency as currently, `hpack` casts everything to a `str` first before converting back to bytes: https://github.com/python-hyper/hpack/blob/02afcab28ca56eb5259904fd414baa89e9f50266/src/hpack/hpack.py#L150-L151

Author: BYK