Save has_one associations only if record has changes

Prevents save related callbacks such as `after_commit` being
triggered when `has_one` objects are already persisted and have no
changes.
Only use BINARY for mysql case sensitive uniqueness check when column has a case insensitive collation.
Add tip about using RDoc's --pipe command to quickly test the output of
a piece of documentation.

This can be useful for trying out combinations of markup and code, as
discussed in rails/rails@225bcad
change 'assert !' to 'assert_not' in guides [ci skip]
Merge pull request #14120 from gregmolnar/guides

change 'assert !' to 'assert_not' in guides [ci skip]
Revert "speed up the collection proxy reader method, but slow down the constructor"

This reverts commit f9e4c3c7c0c4152b62fe9202a9d12262884bb118.

[fixes #14116]
Add regression test for collection proxy reader

[related #14116]
[skip ci] Fix the javascript path in guide for asset modules
Merge branch 'master' of github.com:rails/docrails
Merge pull request #14124 from arthurnn/fix_14116

Fix collection proxy exists? regression
Merge pull request #14086 from jingweno/master

Fix the javascript path in guide for asset modules [skip ci]
Check if the `request` variable isn't nil when calling render_to_string

closes #14125
Merge pull request #14129 from joankaradimov/fix-render-to-string

Fix `render_to_string` when called via manually instantiated controllers
Revert "context in validation goes through has many relationship"

This reverts commit 5e3d466d52fa4e9a42c3a1f8773a7c31da875e48.
adding a test for #14106
[ci skip] update guides to new version of rails 3.2.x
Merge pull request #14131 from kuldeepaggarwal/upgrade-rails-version

update guides to new version of rails 3.2.x, i.e. 3.2.17 [skip ci]
releasing Rails no longer needs triggering docs generation by hand [ci skip]

GitHub is now able to notify tag creation, I have configured the webhook
for the docs server to receive that event. Therefore, we no longer need
to trigger docs generation manually when releasing.
Prevent foreign_key_for? from type casting all attributes
Revert "Only lookup `config.log_level` for stdlib `::Logger`. Closes #11665."

This reverts commit e0a521cfcd13e4d1f0ae8ab96004289e1c020f71.

Conflicts:
	railties/CHANGELOG.md

We expect loggers to quack like stdlib logger. If log4r needs different
level= assignment, using a Logger-quacking wrapper is the way to do it.

Fixes #14114.