Allow specifying a default value in overloaded properties
Update guides to mention 3.2.18.

And reword things a little to mention the specific patch release less.
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 #15426 from oggy/doc-3.2.18

Update guides to mention 3.2.18. [ci skip]
Merge pull request #15354 from sgrif/sg-properties-default

Allow specifying a default value in overloaded properties
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