Merge pull request #15449 from aditya-kapoor/correct-debug-doc

[ci skip] correct doc for ActionView::Helpers::DebugHelper
Removed 'Caching with Rails' guide from WIP, this guide isn't ready to
be published and requires more attention. [ci skip]
config.assets.digests -> config.assets.digest

in Asset Pipeline guide.

[skip ci]
Merge pull request #15456 from jonatack/patch-7

config.assets.digests -> config.assets.digest
Merge branch 'master' of github.com:rails/docrails
Create Changing Columns on migrations guides

[ci skip]
Add Total of grouped items to AR query interface on guides

[ci skip]
Added missing test case for migrations when using sql structure
Merge pull request #14881 from robin850/patch-19

Swallow bundler exceptions when running tests
Don't change values in `@raw_attributes` during serialization

During `init_with`, the attributes given to the coder will be placed
into `@raw_attributes`. As such, we should read from `@raw_attributes`
when encoding, rather than `@attributes`, which has been type cast.
Merge pull request #15432 from sgrif/sg-coder-type-casting

Don't change values in `@raw_attributes` during serialization
Remove most code related to serialized properties

Nearly completely implemented in terms of custom properties.
`_before_type_cast` now stores the raw serialized string consistently,
which removes the need to keep track of "state". The following is now
consistently true:

- `model.serialized == model.reload.serialized`
- A model can be dumped and loaded infinitely without changing
- A model can be saved and reloaded infinitely without changing
Test the serialized types of virtual columns in XML

The previous tests were passing, because nothing ever looked at the
generated XML. What was previously being generated was
`<firstname type="NilClass">...`, which is not consistent with all other
cases where there is not a known type.
Refactor XML serializer to not depend on `serialized_attributes`
Merge pull request #15435 from sgrif/sg-rm-serialization

Remove most code related to serialized properties
Update documentation to reflect unsubscription with name.
Merge pull request #15459 from maurogeorge/add-total-grouped-items

Add Total of grouped items to Active Record query interface on guides [ci skip]
Remove docs regarding grouping multiple values, as we should avoid
duplicating API reference in guides. Also :scissors: [ci skip]
Merge pull request #15457 from maurogeorge/add-changing-columns

Create Changing Columns on migrations guides [ci skip]
:nail_care: for migrations guide from #15457 [ci skip]