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
Reduce number of subscriptions created.
[ci skip] Add changelog for PR #14899

The changelog entry for #14899 was missing.
Merge pull request #16095 from eileencodes/update-changelog-for-pr-14899

Add changelog for PR #14899
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]
[ci skip] Document millisecond precision change in ActiveSupport::TimeWithZone#as_json
Include missing module in tag_helper

Since 6857415187810f1289068a448268264d0cf0844f we are using #safe_join to
join the content when an Array is given, so we must include the dependent
module here to make sure it's available when this module is used alone.

This was making Simple Form tests to fail with current master due to the
missing dependency.
Merge pull request #16091 from tgxworld/reduce_creation_of_subscribers

Reduce number of subscriptions created.
Merge pull request #16113 from egilburg/rails_4_1_upgrade_guide_json_time_with_zone_precision

[ci skip] Document millisecond precision change in ActiveSupport::TimeWithZone#as_json