Implement `If-Match` HTTP header

[Syndesi]

This issue is a follow up from #88 .

See also https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Match .

• [x] Add If-Match-header support.
• [x] Add unit tests.
• [x] Add feature tests.
  • [x] Add new test case, general.if-match.
  • [x] Test with ETag, without ETag, wrong ETag and weak ETag.
  • [x] Test with ETags, where the header is written differently (lowercase, uppercase, mixed, ...).
  • [x] Check If-Match-header against PUT, PATCH and DELETE endpoints.
• [x] Add documentation.