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

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.

Swallow bundler exceptions when running tests

config.assets.digests -> config.assets.digest

in Asset Pipeline guide.

[skip ci]

be published and requires more attention. [ci skip]

[ci skip] correct doc for ActionView::Helpers::DebugHelper

[ci skip] Correct output of Hash#symbolize_keys

Clear all caches calculated based on `@columns` when `@columns` changes

[ci skip] Correct documentation of HashWithIndifferentAccess#dup

Update documentation for setting asset_host to a Proc [ci skip]

Update documentation for setting asset_host to a Proc, mention that the request parameter might not be supplied [ci skip]

Add missing test case for writing unknown attributes

HashWithIndifferentAccess#dup doc [ci skip]

The phrase "exact copy" in the existing docmentation is somewhat
misleading.

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).

Rename attribute related instance variables to better express intent

Allow specifying a default value in overloaded properties

Update guides to mention 3.2.18. [ci skip]

`@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.

And reword things a little to mention the specific patch release less.

Speed-up TimeWithZone to String conversion

[ci skip]

We only introduce new deprecations at Minor releases.

Update Minitest references in Testing Guide

The Guide references `test/unit`, which has been replaced by `Minitest` in
all versions of Ruby supported by Rails. The following updates were
performed:

* The superclass of `ActiveSupport::TestCase` is no longer
  `MiniTest::Unit::TestCase`, but `Minitest::Test` [1].

* The preferred spelling is `Minitest`, not `MiniTest` [2].

* For a method to be a test in Minitest, its name must start with `test_`,
  not only `test` [3].

* Explanations about `test/unit` in Ruby 1.8 were removed in favor of more
  up-to-date information on `Minitest`.

[1]: https://github.com/rails/rails/blob/6c2b569660c9216595d02ce6412e3bdcc7293317/activesupport/lib/active_support/test_case.rb#L18
[2]: https://github.com/seattlerb/minitest/blob/d5d43cef9a3fd4a0eea972dde125ed5ba1ddb821/lib/minitest.rb#L9
[3]: https://github.com/seattlerb/minitest/blob/d5d43cef9a3fd4a0eea972dde125ed5ba1ddb821/lib/minitest/test.rb#L62

This removes the case statement in `SchemaDumper` and gives every `Type`
the possibility to control the SchemaDumper default value output.

/cc @sgrif

This is an intermediate solution. It is related to the refactoring @sgrif
is making and will change in the future.