`ActionDispatch::Head` was replaced by `Rack::Head`. Closes #14191.

See 449039a86d802871b707dfb51ac1ed96d53526f9 for the original commit.
Add missing parantheses in index_exists?
Point master changelogs to 4-1-stable branch

Remove 4-1 related entries from master [ci skip]
Merge pull request #14193 from ys/patch-1

Add missing parantheses in index_exists?
:scissors: [ci skip]
Optimize getting started guide images, and use png instead of jpg

We noticed the rails gem increased in size between the beta and rc
versions, and the reason is that some guides images were updated but not
properly optimized, and one image changed from png to jpg, way bigger.

Converting the jpg image back to png, and using ImageOptim to optimize
all the images for web view, improves the situation [ci skip].

---

In current master/4-1-stable:

    508K  guides/assets/images/getting_started/

In v4.1.0.beta1:

    268K  guides/assets/images/getting_started/

In 4-0-stable

    328K  guides/assets/images/getting_started/

After the patch:

    260K  guides/assets/images/getting_started/
Skip test_migrate_revert_add_index_with_name if databases
do not allow to create duplicate indexes on the same columns
Merge pull request #14177 from yahonda/skip_tests_if_duplicate_index_not_allowed

Skip test_migrate_revert_add_index_with_name for some databases