Prevent state leak.
Remove redundant code.
remove unnecessary substitution for space in ActionView::Helpers#debug
Always update counter caches in memory when adding records

Before, calling `size` would only work if it skipped the cache, and
would return a different result from the cache, but only if:

- The association was previously loaded
- Or you called size previously
- But only if the size was 0 when you called it

This ensures that the counter is appropriately updated in memory.
Don't save through records twice

If the through record gets created in an `after_create` hook that is
defined before the association is defined (therefore after its
`after_create` hook) get saved twice. This ensures that the through
records are created only once, regardless of the order of the hooks.
Don't include inheritance column in the through_scope_attributes
Don't use `Column` for type casting in Relation calculations
Don't use column object when calculating type cast defaults

Moves towards removing type casting knowledge from the column entirely
Don't use column object for type casting in `quoting`

We're never going to be able to use the attribute object here, however,
so let's just accept the ugly demeter violation here for now.

Remove test cases which were either redundant with other tests in the
file, or were actually testing the type objects (which are tested
elsewhere)
Don't rely on the column for type casting reflections
Create custom errors page on ActionController guides

[ci skip]
Remove unused block parameter from #gsub call in AM::MailHelper#block_format
Relpace `=~ Regexp.new str` with `.include? str` in AC::Base#_valid_action_name?

Because it is more natural way to test substring inclusion. Also, in
this particular case it is much faster.

In general, using `Regexp.new str` for such kind of things is dangerous.
The string must be escaped, unless you know what you're doing. Example:

    Regexp.new "\\" # HELLO WINDOWS
    # RegexpError: too short escape sequence: /\/

The right way to do this is escape the string

    Regexp.new Regexp.escape "\\"
    # => /\\/

Here is the benchmark showing how faster `include?` call is.

```
require 'benchmark/ips'

Benchmark.ips do |x|
  x.report('include?') { !"index".to_s.include? File::SEPARATOR }
  x.report('   !~   ') { "index" !~ Regexp.new(File::SEPARATOR) }
end

__END__
Calculating -------------------------------------
            include?     75754 i/100ms
               !~        21089 i/100ms
-------------------------------------------------
            include?  3172882.3 (±4.5%) i/s -   15832586 in   5.000659s
               !~      322918.8 (±8.6%) i/s -    1602764 in   4.999509s
```

Extra `.to_s` call is needed to handle the case when `action_name` is
`nil`. If it is omitted, some tests fail.
Further simplify `changed?` conditional for numeric types

`Type::Integer.new.type_cast('') # => nil`, we do not need a special
case to handle this, `nil => ''` already returns false. The only case we
need to handle is `0 => 'wibble'` should be changed, while `0 => '0'`
should not.
Add `logger.debug?` guard to `ActionMailer::LogSubscriber#process`

*TLDR*: The method is 4 times faster when log level is higher than DEBUG.
Also, the other two methods, `#deliver` and `#receive` have similar guard statements,
so this commit adds some symmetry to the code.

This is probably not the most critical part of ActionMailer in terms
of performance, but here are some benchmarks:

```
require 'benchmark/ips'
require 'action_mailer'

event = ActiveSupport::Notifications::Event.new(
            'process.action_mailer',
            Time.now,
            Time.now,
            'bf4e2b36ce085fd35b24',
            { mailer: "UserMailer", action: :welcome }
        )

ActionMailer::Base.logger = ActiveSupport::Logger.new '/dev/null'

subscriber = ActionMailer::LogSubscriber.new
def subscriber.process_with_guard(event)
  return unless logger.debug?
  mailer = event.payload[:mailer]
  action = event.payload[:action]
  debug("\n#{mailer}##{action}: processed outbound mail in #{event.duration.round(1)}ms")
end

Benchmark.ips do |r|
  ActionMailer::Base.logger.level = ::Logger::Severity::INFO
  r.report('no guard') { subscriber.process(event) }
  r.report('   guard') { subscriber.process_with_guard(event) }
end

__END__
Calculating -------------------------------------
            no guard      9640 i/100ms
               guard     38381 i/100ms
-------------------------------------------------
            no guard   169166.9 (±10.2%) i/s -     838680 in   5.007262s
               guard   728184.9 (±9.6%) i/s -    3607814 in   4.999218s
```

When log level is DEBUG, the _guarded_ method is a bit slower. This is a
good tradeoff for 4x improvement in production mode.

```
Benchmark.ips do |r|
  ActionMailer::Base.logger.level = ::Logger::Severity::DEBUG
  r.report('no guard') { subscriber.process(event) }
  r.report('   guard') { subscriber.process_with_guard(event) }
end

__END__
Calculating -------------------------------------
            no guard      4970 i/100ms
               guard      4564 i/100ms
-------------------------------------------------
            no guard    55617.4 (±3.5%) i/s -     278320 in   5.010523s
               guard    49452.1 (±5.6%) i/s -     251020 in   5.093358s

```
Merge pull request #15816 from DNNX/log-subscriber-add-debug-check

Add `logger.debug?` guard to `ActionMailer::LogSubscriber#process`
Merge pull request #15815 from DNNX/mail-helper-refactoring

Remove unused block parameter from #gsub call in AM::MailHelper#block_format
Merge pull request #15813 from DNNX/valid-action-name-refactoring

Relpace `=~ Regexp.new str` with `.include? str` in `_valid_action_name?`
[ci skip] Fix typo in 4.1 upgrade/flash structure
Merge pull request #15817 from zoombody/patch-1

Fix typo in 4.1 upgrade/flash structure