Implement `If-None-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-None-Match .

• [x] Implement feature.
  • [x] Make sure that Get and Head requests return 304.
• [x] Unit tests.
• [x] Feature tests.
  • [x] Fix missing put and patch element tests for controller example generation :(
• [x] Add documentation.
  • [x] Add 412 response examples on all affected endpoints.
  • [x] Update all affected charts.