removed unnecessary test case

Remove `DatabaseStatements#case_sensitive_equality_operator`. It has been deprecated already.

#type_cast - improve performance & readability

removed unnecessary test case and improved test case for belongs_to having invalid  options

fix mysql version check in rename_index

also clarify native rename_index support is >= 5.7, not > 5.7

Make it prominent that after_commit and after_rollback swallow exceptions.

The section that mentions after_commit and after_rollback will swallow exceptions should me made clear. Made that section to be a  instead of a regular paragraph.

implement rename_index natively for MySQL > 5.7

Fix an activesupport inflector test changing the inflector but not reverting all its changes afterwards

/cc @fxn

Extract **notable changes**, **deprecations** and **removals** from
each CHANGELOG.

I tried to reference the commits and pull requests for new features
and deprecations.

In the process I also made some minor changes to the CHANGELOGS.

The 4_1_release_notes guide is declared WIP.

because it is only used in
`activerecord/test/cases/adapters/oracle/synonym_test.rb`

See rails/rails#13054

Remove leftover Oracle tests.

fixed typo `destory` => `destroy` [ci skip]

Deprecate ActiveSupport::TestCase.describe

Provide message to define ActiveSupport::TestCase.name instead.
Allow calling describe with a block, which Minitest::Spec does.

[ci skip] removed singulars section from classify doc

Conflicts:
	activesupport/CHANGELOG.md

unnecessary checking of `size` with `second regex` if matched with former regex

Link for "New Post" is added in Getting Started guide [ci skip]

Dropped yajl from Gemfile

Requires JSON gem version 1.7.7 or above as it contains an important security fix.

[ci skip]removed obsolete information about `options` parameter in create method

[ci skip] used new syntax for scopes