Upgraded rack
As Rack has some non backwards compatible changes added required
modifications to keep behaviour in rails close to same as before.

Also modified generators to include rack/rack for not yet released
version of rack
Since upgrading rack we can remove unnecessary string encodings
https://github.com/rack/rack/commit/5a5aee36
Merge pull request #13999 from jamox/update_rack

This updates rails to use edge rack
correct markdown usage [ci skip]
Merge pull request #16097 from nishantmodak/correct_md

correct markdown usage [ci skip]
Fix weird comment. [CI SKIP]
[ci skip] Fix 4.2 release notes rendered display.
Merge pull request #16100 from tgxworld/fix_comment

Use proper possessive punctuation [ci skip]
Merge pull request #16102 from JuanitoFatas/format-fixes

[ci skip] Fix 4.2 release notes rendered display.
[ci skip] Fix 4.2 release notes list items. [Matthew Draper & Juanito Fatas]
Merge pull request #16103 from JuanitoFatas/doc/patch-001

[ci skip] Fix 4.2 release notes list items. [Matthew Draper & Juanito Fatas]