draft-ietf-httpbis-unencoded-digest-02

Editorial suggestions from Roberto Polli (#3348)

Most (but not all :smile: ) suggestions from @ioggstream. Some of these
triggered memories from RFC editor feedback on 9530 so applying now
should ensure some good consistency

- **Add some imports in the conventions and definitions**
- **omit one -> omit the field**
- **gzip example wording**
- **hashing of entire selected representation data**
- **sprinkle some s/coding/codings around**
- **s/body/content**
- **Improve HTTP in examples to avoid a call from the protocol police**