Skip if the association was already loaded.
remove unwanted `to_sym` call.
Add a public API to allow users to specify column types

As a result of all of the refactoring that's been done, it's now
possible for us to define a public API to allow users to specify
behavior. This is an initial implementation so that I can work off of it
in smaller pieces for additional features/refactorings.

The current behavior will continue to stay the same, though I'd like to
refactor towards the automatic schema detection being built off of this
API, and add the ability to opt out of automatic schema detection.

Use cases:

- We can deprecate a lot of the edge cases around types, now that there
  is an alternate path for users who wish to maintain the same behavior.
- I intend to refactor serialized columns to be built on top of this
  API.
- Gem and library maintainers are able to interact with `ActiveRecord`
  at a slightly lower level in a more stable way.
- Interesting ability to reverse the work flow of adding to the schema.
  Model can become the single source of truth for the structure. We can
  compare that to what the database says the schema is, diff them, and
  generate a migration.
Replace double quotes with single quotes while adding an entry into Gemfile
Add description of ActionMailer::TestHelper module [ci skip]
Use "Action Mailer" as canonical name in documentation [ci skip]

See also:
https://github.com/rails/rails/commit/3a297f8db75b172882bb3c2f9440d0c7365b8b11#commitcomment-6451708
Remove unnecessary branch from quoting in Mysql

This is already the behavior for `BigDecimal` in the abstract adapter.
Pass through AM documentation re: 02d048d [ci skip]
Merge pull request #15284 from sgrif/sg-properties

Add a public API to allow users to specify column types
Typo in AR CHANGELOG [ci skip]
Merge pull request #15353 from sgrif/sg-mysql-condition

Remove unnecessary branch from quoting in Mysql
Merge pull request #15301 from zzak/issue_15212

Skip if the association was already loaded.
Add test case for b13d260
Move test to the proper file

All preload tests are in the eager_test file
Fix syntax error from 5eae77c
Remove `require 'active_support'` from individual modules

Let users require `active_support` before loading any ActiveSupport modules
http://guides.rubyonrails.org/active_support_core_extensions.html
Fix redefine a has_and_belongs_to_many inside inherited class

After ad7b5efb55bcc2e0ccd3e7f22a81e984df8676d1, 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]
Merge pull request #15356 from deeeki/activesupport_loading

Remove `require 'active_support'` from individual modules
Merge pull request #15327 from alexbel/replace_double_quotes_with_single_quotes

Replace double quotes with single quotes while adding an entry into Gemfile
Deprecate decimal columns being automatically treated as integers

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.