asset guide, match application.rb snippets to the generated file [ci skip]

`if defined?(Bundler)` is no longer used in our current application.rb
cleanup, switch arguments in assert_equal calls
cleanup, assert on warning from postgres adapter
Fixes bug 10628.
Eliminate minitest warnings

https://github.com/seattlerb/minitest/commit/9a57c520ceac76abfe6105866f8548a94eb357b6#L15R8
Extract AR::Core#inspect and AR::Core.inspect's test code from base_test.rb

The methods got moved to core.rb in commit
b2c9ce341a1c907041f55461aefebb0321280cb5, but the tests never did.
Extract AR::AttributeMethods#attribute_for_inspect's test code out from base_test.rb

The method itself got extracted out from ActiveRecored::Base in commit
ceb33f84933639d3b61aac62e5e71fd087ab65ed, but the test code never did.
Extract AR::Persistence#becomes's test code out from base_test.rb

The method got extracted out from AR::Base in commit
d916c62cfc7c59ab6411407a05b946d3dd7535e9, but the tests never did.
Merge pull request #10714 from tkhr/extract_test_from_base_test.rb_in_AR

Extract tests code out from AR's base_test.rb
remove code duplication
Merge pull request #10725 from neerajdotname/remove_code_duplication

Remove code duplication
Merge pull request #10667 from rubys/minitest-autorun

Eliminate minitest warnings
Require only minitest.

minitest/autorun required minitest/spec and we are avoiding to require
it.
Add CHANGELOG entry for 99860582b2b1c0fc42bf84c52aac57b243d42678
Fix the `:primary_key` option for `has_many` associations.

When removing records from a `has_many` association it used
the `primary_key` defined on the association.

Our test suite didn't fail because on all occurences of `:primary_key`,
the specified column was available in both tables. This prevented the
code from raising an exception but it still behaved badly.

I added a test-case to prevent regressions that failed with:

```
  1) Error:
HasManyAssociationsTest#test_has_many_assignment_with_custom_primary_key:
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: essays.first_name: UPDATE "essays" SET "writer_id" = NULL WHERE "essays"."writer_id" = ? AND "essays"."first_name" IS NULL
```
deprecate Validator#setup (to get rid of a respond_to call). validators do their setup in their constructor now.
Merge branch 'deprecate-validator-setup'

Closes #10716
Merge pull request #10713 from senny/10693_fix_primary_key_option_on_has_many

Fix the `:primary_key` option for `has_many` associations.
Merge pull request #10222 from senny/update_bundler_section_in_assets_guide

docs, adjust the application.rb snippets to match the generated file.

Closes #10449
Add note about upgrading custom routes from `put` to `patch`.