[API] Ignore If-Modified-Since when If-None-Match is present

[RedZapdos123]

Description:

Issue #329 reported that Rage still evaluated If-Modified-Since when If-None-Match was present, so an older If-Modified-Since could force a 200 OK even when If-None-Match already matched.

This PR updates Rage::Request#fresh? to give If-None-Match precedence when it is present and only fall back to If-Modified-Since when no If-None-Match header is sent.

It also adds regression tests for the matching If-None-Match + stale If-Modified-Since case and for the path where the response only sets an ETag.

Closes #329.

Checklist:

• I have added relevant regression test cases for this fix.
• I have run linting checks in WSL using bundle exec rubocop --except Layout/EndOfLine lib/rage/request.rb spec/controller/api/conditional_get_spec.rb.
• I have run focused tests in WSL using bundle exec rspec spec/controller/api/conditional_get_spec.rb.
• I have run broader regression checks in WSL using bundle exec rspec spec/controller/api.

Before the fix:

After the fix: