Move I18n.locale setting into setup and teardown.

Add with_default_charset helper.

The solution presented in this patch is not efficient. We should replace it
in the near future. The following needs to be worked out:
  * Is `@attributes` storing the Ruby or SQL representation?
  * `cacheable_column?` is broken but `hstore` and `json` rely on that behavior

Refs #15369.

/cc @sgrif @rafaelfranca

Remove AR Properties from the public API

Making this part of the public API was premature, let's make it private
again while I continue to work on the surrounding code.

rake railties:install:migrations respects the order of railties

Add test case for clear mappings

This is the continuation of the work started at
9d62e048

* constraints:
  rm reset_parameters because we automatically do it from 9ca4839a
  move path_parameter encoding check to the request object
  dispatcher doesn't need `call` anymore
  call `serve` with the request on dispatchers
  constraints class does not need the request class anymore
  give all endpoints a superclass
  skip the build business if the stack is empty
  stop hardcoding path_parameters and get it from the request
  we do not need to cache rack_app
  a redirect is not a dispatcher by definition, so eliminate test
  push is_a check up to where the Constraints object is allocated
  pass the request object to the application
  pass a request to `matches?` so we can avoid creating excess requests
  nothing is passed to `rack_app` anymore, so rm the params
  one fewer is_a check
  Constraints#app should never return another Constraints object, so switch to if statement
  eliminate dispatcher is_a checks
  push is_a?(Dispatcher) check in to one place
  Always construct route objects with Constraint objects

Conflicts:
	actionpack/lib/action_controller/metal.rb

Test Case - Change method name

Silence deprecation warning in test

Missed a case that only occured for mysql/mysql2

Added partial days support to `DateTime`'s `advance` method.

Call get_routes_as_head only on HEAD requests.

Railties need to be used without having to require any dependecy first.

Correctly alias table names when joining more than once

Deprecate decimal columns being automatically treated as integers

This PR fixes #8930 and some stuff from #8985

Fix redefine a has_and_belongs_to_many inside inherited class

remove unwanted `to_sym` call.

With ActiveRecord::Properties, we now have a reasonable path for users
to continue to keep this behavior if they want it. This is an edge case
that has added a lot of complexity to the code base.

Replace double quotes with single quotes while adding an entry into Gemfile

Remove `require 'active_support'` from individual modules

After ad7b5efb, which changed how
has_an_belongs_to_many used to work, we start raising an error when
redefining the same has_an_belongs_to_many association. This commits fix
that regression.

[Fixes #14983]

Let users require `active_support` before loading any ActiveSupport modules
http://guides.rubyonrails.org/active_support_core_extensions.html

All preload tests are in the eager_test file