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
Swallow bundler exceptions when running tests

Let's avoid useless output when testing stuff related to Bundler.
:scissors:
Fix Baseclass becomes! subclass.
Remove duplicated HashWithIndifferentAccess#with_indifferent_access.
[ci skip] Fix Compiler's Github link
implement ActiveRecord::Base#pretty_print + changelog
[ci skip] add created_at and updated_at to example
document generators hidden_namespaces
Fixed `ActiveSupport::TimeWithZone#-` so precision is not unnecessarily lost

When working with objects with a nanosecond component, the `-` method may
unnecessarily cause loss of precision.

`ActiveSupport::TimeWithZone#-` should return the same result as if we were
using `Time#-`:

    Time.now.end_of_day - Time.now.beginning_of_day #=> 86399.999999999

Before:

    Time.zone.now.end_of_day.nsec #=> 999999999
    Time.zone.now.end_of_day - Time.zone.now.beginning_of_day #=> 86400.0

After:

    Time.zone.now.end_of_day - Time.zone.now.beginning_of_day
    #=> 86399.999999999
Clear all caches calculated based on `@columns` when `@columns` changes
Update documentation for setting asset_host to a Proc, mention that the request parameter might not be supplied [ci skip]
[ci skip] Correct documentation of HashWithIndifferentAccess#dup
Merge pull request #15445 from krautcomputing/fix_asset_host_docs

Update documentation for setting asset_host to a Proc [ci skip]
Merge pull request #15446 from akshay-vishnoi/doc_changes

[ci skip] Correct documentation of HashWithIndifferentAccess#dup
Tiny formatting fix in AS change log [ci skip]
Test `except!` in other cases too
Merge pull request #15434 from sgrif/sg-clear-caches

Clear all caches calculated based on `@columns` when `@columns` changes
[ci skip] Correct output of Hash#symbolize_keys