Fine tune wording of the requirement for including Content-Type in the response

Extracting from: https://github.com/solid/specification/pull/561#discussion_r1305914478

In short, we want to make sure that the work makes it clear that it also applies when responding to HEAD requests.

@woutermont proposed

Server MUST generate a Content-Type header field in a message that contains a representation, even if that representation only consists of representation metadata, as in a HEAD request.

I think this is clear and concise. Maybe adjusting the last sentence to , as in a response to HEAD request

Since the wording in the spec includes nuances like

In a response to a HEAD request, the representation header fields describe the representation data that would have been enclosed in the content if the same request had been a GET

we should triple-check if we use all the terms strictly as intended by the RFC.

Locate the specification text containing the Content-Type requirement and the cited wording about responses to HEAD requests. Compare the terminology with the referenced RFC discussion, then update the requirement so its scope and HEAD-request applicability are explicit and technically consistent.