Fix bug, when ':dependent => :destroy' option violates foreign key constraints, issue #12380
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.
[ci skip] correct output for asset_helper methods
    see cc255d3
Detect in-place modifications on Strings
Added missing colon in asset manifest JSON.
Add rails:update to the upgrading guides

[ci skip]
Add warning for `rescue_from Exception` and `rescue_from StandardError`

[ci skip]
Replace x.sort_by!.select! with x.select!.sort_by!

The latter has the same speed as the former in the worst case
and faster in general, because it is always better to sort less items.

Unfortunately, `routes.select!{...}.sort_by!{...}` is not possible here
because `select!` returns `nil`, so select! and sort! must be done
in two steps.
Return a null object from `AttributeSet#[]`
[ci skip] Add Tip for change_column_null and change_column_default in guides
Merge pull request #15624 from aditya-kapoor/add-warnings-on-guides

[ci skip] Add Tip for change_column_null and change_column_default in guides
:nail_care: wrap the tip from #15624 at 80 chars [ci skip]
Make the isolated tests run on JRuby

As there is no forking on JRuby, we need to spawn sub-processes to make
the tests run in isolation.

Previously, we were defining globally env variables and running the test
file through backticks and delete these variables once the test ran.

Now, we simply rely on IO.popen as this is cross-platform and the env
variables are available during the child-process execution only so there
are no race conditions.

[Ben Browning & Robin Dupret]
Move `attributes_before_type_cast` to `AttributeSet`
Move `attributes` to the `AttributeSet` object.
Add missing test cases for `attribute_method?`
Change double quotes to single quotes in guide
Improve consistency
`only_integer` of `NumericalityValidator` now allows procs and symbols
Use with_parse_json_times helper in tests.
Avoid hardcoded magic number in test teardown.