Upgrade guide pass. [CI SKIP]
Avoid duplicating routes for HEAD requests.

Follow up to rails#15321

Instead of duplicating the routes, we will first match the HEAD request to
HEAD routes. If no match is found, we will then map the HEAD request to
GET routes.
Improve router test.

We should assert that routes will not be recognized if the verbs do
not match.
Merge pull request #15443 from tgxworld/preload_head_routes

Map HEAD requests to GET routes instead of duplicating GET routes.
Fix some issues in 4.2 release notes.

Remove duplicate line in Railties.

[ci skip]
Merge pull request #16608 from GRoguelon/fix_issues_in_4_2_release_notes

Fix some issues in 4.2 release notes. [ci skip]
Merge pull request #16607 from tgxworld/upgrading_guide_pass

Upgrade guide pass. [CI SKIP]
[ci skip] Clarify Action Mailer/Active Job usage note
Merge pull request #16612 from loganhasson/master

[ci skip] Clarify Action Mailer/Active Job usage note