This PR improves our handling of HTTP message body encodings:
- The unaltered message body is now accessible as `.raw_content`
- The "content-encoding"-decoded content (i.e. gzip removed) content
is not `.content`, as this is what we want in 99% of the cases.
- `.text` now provides the "content-encoding"-decoded and then
"content-type charset"-decoded message body.
- The decoded values for `.content` and `.text` are cached,
so that repeated access and `x.text = x.text` is cheap.
- The `decoded()` decorator is now deprecated, as we can now just use
`.content`. Similarly `HTTPMessage.get_decoded_content()` is
deprecated.
