Digest acts on the representation-data, not the message body

[Roberto Polli]

I expect

Digest is calculated on the representation-data of the selected representation

Instead

the spec ties Digest to the message body

Notes

See:

• https://github.com/httpwg/http-extensions/blob/master/draft-ietf-httpbis-digest-headers.md#this-proposal
• https://tools.ietf.org/html/rfc3230#section-3 definition of instance