Fix bug, when ':dependent => :destroy' option violates foreign key constraints, issue #12380
Use NameError#name to assert raised error.

This makes the test compatible with other Ruby implementations, which
may implement error messages differently.
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.
Upgraded rack
As Rack has some non backwards compatible changes added required
modifications to keep behaviour in rails close to same as before.

Also modified generators to include rack/rack for not yet released
version of rack
Since upgrading rack we can remove unnecessary string encodings
https://github.com/rack/rack/commit/5a5aee36
[ci skip] correct output for asset_helper methods
    see cc255d3
Formatting fix in AS changelog [ci skip]
Revert "Formatting fix in AS changelog [ci skip]"

This reverts commit cede0363830e28e73feafc662eccae7d1db526bb.

Reason: CHANGELOG should not be updated on docrails
Detect in-place modifications on Strings
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.
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`
`only_integer` of `NumericalityValidator` now allows procs and symbols
Encapsulate knowledge of type objects on `ActiveRecord::Result`

Attempting to reduce the number of places that care about the details of
how type casting occurs. We remove the type casting of the primary key
in `JoinDependecy`, rather than encapsulating it. It was originally
added for consistency with
https://github.com/rails/rails/commit/40898c8c19fa04442fc5f8fb5daf3a8bdb9a1e03#diff-06059df8d3dee3101718fb2c01151ad0R211,
but that conditional was later removed in
https://github.com/rails/rails/commit/d7ddaa530fd1b94e22d745cbaf2e8a5a34ee9734.

What is important is that the same row twice will have the same value
for the primary key, which it will.
Delegate #model_name method to self.class
Add AR::Base#validate! method

Acts same as valid? but raises AR::RecordInvalid exception
if validation fails
Add a deprecation cycle for `NullColumn` from `column_for_attribute`

This is public API, and `simple_form` depends on the `nil` return value.
We need to go through a deprecation cycle to return a null object. If
people want hash access, they can access the hash.
Small typo