Rename attribute related instance variables to better express intent

`@attributes` was actually used for `_before_type_cast` and friends,
while `@attributes_cache` is the type cast version (and caching is the
wrong word there, but I'm working on removing the conditionals around
that). I opted for `@raw_attributes`, because `_before_type_cast` is
also semantically misleading. The values in said hash are in the state
given by the form builder or database, so raw seemed to be a good word.
Merge pull request #15427 from sgrif/sg-rename-attribute-ivars

Rename attribute related instance variables to better express intent
Update some bad test cases for serialized columns

The first case was not testing what the issue mentioned actually was (A
subclass of a class with serialized attributes does not serialize when
they come from the database).

The second case was a bad coder. It would fail if the model was `dup`ed,
or if the the model was loaded from the database and then saved again.

The third case wasn't testing anything that wasn't covered by the second
(and was also a bad coder for the same reasons as the second).
Merge pull request #15431 from sgrif/sg-bad-serialization-test-cases

Update some bad test cases for serialized columns
Add missing test case for writing unknown attributes
HashWithIndifferentAccess#dup doc [ci skip]

The phrase "exact copy" in the existing docmentation is somewhat
misleading.
Merge pull request #15440 from ilikepi/pr_docs_activesupport_hwia_dup

HashWithIndifferentAccess#dup doc [ci skip]
Merge pull request #15437 from sgrif/sg-missing-test-case

Add missing test case for writing unknown attributes
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]
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
Merge pull request #15448 from akshay-vishnoi/doc_changes

[ci skip] Correct output of Hash#symbolize_keys