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
Prevent state leak.
Remove redundant code.
If a counter_cache exists, use it for #empty?
Fixed a regression introduced in 84cf156

84cf156 (PR #15694) introduced a subtle regression. There are actually three
distinct entry points to creating an AR object – via .new (i.e. #initialize),
via #init_with (e.g. from YAML or database queries) and via .allocate.

With the patch in 84cf156, attribute methods and respond_to? will not work
correctly when objects are allocated directly, without going through either

The reason this test case didn't catch the regression was that the `Topic`
class is shared between test cases, so by the time this test case is ran the
attribute methods are very likely to be defined.

Switching to use a fresh anonymous class in the test to ensure we surface this
problem in the future.
[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
Added missing colon in asset manifest JSON.
Add to guides app and helper objects on command line

[ci skip]
Add rails:update to the upgrading guides

[ci skip]
Move to_param to to_query, also Improve tests
Add warning for `rescue_from Exception` and `rescue_from StandardError`

[ci skip]
Merge pull request #15537 from tgxworld/fix_state_leak

Fix state leak.
Merge pull request #15809 from maurogeorge/guides-console-app

Add to guides app and helper objects on command line