Moved explanation of `--mountable` option up

The example code uses `--mountable`. In previous version this was followed by an explanation of the `--full` option, which doesn't make sense. Later on in the text this is better explained. I moved this explanation up without repeating too much.
skip migration check if adapter doesn't support it
Prevent state leak.
Remove redundant code.
Move cleanup into teardown, delete obsolete code.
Add test cases for Hash#asset_valid_keys
remove unnecessary substitution for space in ActionView::Helpers#debug
Add an option to skip installation of Turbolinks
/mysql/i -> MySQL, Spell correct in continuation to #15555
[ci skip] Use `an` for SQL
Change the deprecation warning on `serialized_attributes`

to "without replacement"
Merge pull request #15741 from sgrif/sg-update-deprecation-message

Change the deprecation warning on `serialized_attributes`
Remove unused parameter.
Remove unused param 'separators' from RouteSet#build_path
Merge pull request #15745 from mmozuras/remove_unused_separators_param

Remove unused param 'separators' from RouteSet#build_path
Change Http::Cache::SPECIAL_KEYS from Array to Set

Slightly improves performance, for example, a simple benchmark:

```ruby
require 'benchmark/ips'
require 'set'

SPECIAL_KEYS = %w[extras no-cache max-age public must-revalidate]
SPECIAL_KEYS_SET = Set.new(SPECIAL_KEYS)
directive = 'must-revalidate'

Benchmark.ips do |x|
  x.report('array') { SPECIAL_KEYS.include?(directive) }
  x.report('set') { SPECIAL_KEYS_SET.include?(directive) }
end
```

Output:

```
-------------------------------------
   array     67926 i/100ms
     set     74054 i/100ms
-------------------------------------
   array  2318423.4 (±2.8%) i/s -   11615346 in   5.014899s
     set  3387981.8 (±4.7%) i/s -   16958366 in   5.019355s
```
Correct typo, add test for validates_absence_of, correct method names
Merge pull request #15748 from akshay-vishnoi/activemodel

Correct typo, add test for validates_absence_of
Merge pull request #15723 from akshay-vishnoi/sql-correction

[ci skip] Use `an` for SQL
Merge pull request #15722 from akshay-vishnoi/spell-correct

[ci skip] /mysql/i -> MySQL, Spell correct in continuation to #15555