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The Guide references `test/unit`, which has been replaced by `Minitest` in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.

Fixed #columns_for_distinct of postgresql adapter

Conflicts:
	activerecord/CHANGELOG.md

Replacements:

   5.ago   => 5.seconds.ago
   5.until => 5.seconds.until
   5.since => 5.seconds.since
   5.from_now => 5.seconds.from_now

The removed tests does not affect coverage – we have equivalent test cases in
the tests for `AS::Duration`.

See #12389 for the history and rationale behind this.

`bin/setup` script to bootstrap applications.

[ci skip] Document Rails' shifted semver.

* mapper: (34 commits)
  no more is_a checks on instantiation
  Path::Pattern is instantiated internally, so make the contructor require a strexp object
  Strexp#names is only used in a test, so rm
  pass the parsed path from mapper to the Strexp
  add an alternate constructor to Strexp that takes a string
  ask the strexp for the ast
  remove dead code
  disconnect path from the instance
  reuse the ast we already made
  use a parser to extract the group parts from the path
  pass the parsed parameters through the methods so we don't reparse or require caching code
  "controllers" should be a valid path name
  controllers with slash names are also not supported, so we can reuse the message
  only validate controllers
  golf down a bit
  only error handling between controller and action is the same
  add a test for controllers without colons
  move nil check to a method that yields to a block if the value is not nil
  translate action / controller to the desired object
  only one nil check on the action variable
  ...

[ci skip] Include ActiveModel::Model in a class instead of inheriting

require caching code