Fix ETag Handling

[no-reply]

We have a few problems with ETag handling:

• RFC 7232 sec. 3.1 requires us to use strong validation on If-Match, we use weak.
• Even our weak validation is incorrect; see RFC 7232 sec. 2.3.2.
• If we use strong validation, clients will never be able to use If-Match (since we don't generate strong ETags).

Discussion of this issue within the LDP WG is in this thread.

[no-reply]

The restriction referred to in the email thread linked above was added in https://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-24. I'm unsure whether the LDP WG ever discussed this development.

[no-reply]

The proposed fix is to fail If-Match (Precondition Failed) for all checks.