9.1.4 Response Code 204 No Content: rephrase second paragraph

The second paragraph of section 9.1.4 is a quote from RFC7231 and somewhat hard to parse. Replace it with text that expresses its intention: send an ETag only if the client can reasonably "know" the current entity representation that has not been sent in the response:

(1) For a PUT request: if the response body of a corresponding 200 response would have been identical to the request body, i.e. no server-side modification of values sent in the request body, no "calculated" values.

(2) We could extend this to PATCH requests and if a corresponding 200 response would have consisted of

• values sent in PATCH request body, plus
• server-side values corresponding to ETag sent in If-Match header of PATCH request (for values not sent in PATCH request body)