/cc @zuhao

This adds a regression test for #14411, which was fixed by #15503.

Closes #14411
Closes #14595

Bring type casting behavior of hstore/json in line with serialized

Use a frozen Set instance for CGI_VARIABLES.

The migration numbers were normalized different ways. This left
the task output in an inconsistent state.

Closes #15538.

Closes #15541.

Baseclass becomes! subclass

Also expand the CGI_VARIABLE name listing to multiple lines for cleaner diffs and legibility.

Remove optimization that was required with whiny nils

Whiny nils is no longer a thing, so we no longer need this optimization

Avoid misuse of underscore argument

Per convention, underscore-only argument names should be used for unused parameters.

Reset callbacks after test.

Removed CaptureHelper#flush_output_buffer as it is only used in tests.

document include_blank's usage with a string [ci skip]

Restore Response.default_headers after test.

Avoid hard-coded value in test setup/teardown.

Otherwise the state of callback chain is leaked.

Make sure Mime::Type is not altered after tests.

Fix BC year handling in postgres

Avoid hardcoded value in test setup/teardown.

BC era year is (astronomical year + 1) and starts from 1 BC.

Remove deprecated method ActiveRecord::Migrator.proper_table_name

Fixed `ActiveSupport::TimeWithZone#-` so precision is not unnecessarily lost

Added Two Clarification "Tips" [ci skip]

Fix inverse associations test

It's not very clear exactly what the "params" method is and does, as the current description in the guide is somewhat vague and brief. An example is helpful, especially considering this method is referenced over 30 times in this guide. [ci skip]

It may be confusing to a Rails newbie why "Article" is suddenly being referred to with the "A" being capitalized. Thus far in the guide, the "a" has always been in lowercase. I think it is helpful to point out the reason for this change (especially since not using the capital "A" will result in an error). [ci skip]

`InverseHasManyTests#test_parent_instance_should_be_shared_within_create_block_of_new_child`
was mistakenly the same as
`InverseHasManyTests#test_parent_instance_should_be_shared_within_build_block_of_new_child`.

reviewed: @tenderlove

Emit suggested generator names when not found

When someone types in a generator command it currently outputs all generators. Instead we can attempt to find a subtle mis-spelling by running all generator names through a levenshtein_distance algorithm provided by rubygems. 

So now a failure looks like this:

```ruby
$ rails generate migratioooons
Could not find generator 'migratioooons'. Maybe you meant 'migration' or 'integration_test' or 'generator'
Run `rails generate --help` for more options.
```

If the suggestions are bad we leave the user with the hint to run `rails generate --help` to see all commands.

Follow up to #15438 and #15502.

/cc @sgrif