Fixes #17621. This 5 year old (or older) issue causes validations to fire
when a parent record has `validate: false` option and a child record is
saved. It's not the responsibility of the model to validate an
associated object unless the object was created or modified by the
parent.

Clean up tests related to validations

`assert_nothing_raised` is not benefiting us in these tests
Corrected spelling of "respects"
It's better to use `assert_not_operator` over `assert !r.valid`

Also prevents the word "Model" from linking to the documentation
of ActiveModel::Model because that's not intended.

[ci skip]

We have to specify the `:title` option to really use the
`TitleValidator` defined above.

deprecate Validator#setup (to get rid of a respond_to call). validators do their setup in their constructor now.

As discussed with @josevalim on Ruby Rogues Parley.

Makes it easier to test its objects.

We don't need to define a new method in ActiveMode::Errors for each
validatior.

See
https://github.com/rails/rails/commit/d72a07f1d1478db9daed847eadb35bfd840674f6#commitcomment-2325333

Sometimes, on Mac OS X, programmers accidentally press Option+Space
rather than just Space and don’t see the difference. The problem is
that Option+Space writes a non-breaking space (0XA0) rather than a
normal space (0x20).

This commit removes all the non-breaking spaces inadvertently
introduced in the comments of the code.

ArgumentError is better suited than RuntimeError for this.

Selecting which key extensions to include in active_support/rails
made apparent the systematic usage of Object#in? in the code base.
After some discussion in

    https://github.com/rails/rails/commit/5ea6b0df9a36d033f21b52049426257a4637028d

we decided to remove it and use plain Ruby, which seems enough
for this particular idiom.

In this commit the refactor has been made case by case. Sometimes
include? is the natural alternative, others a simple || is the
way you actually spell the condition in your head, others a case
statement seems more appropriate. I have chosen the one I liked
the most in each case.

This reverts commit fa6d921e.

Reason: Not a fan of such massive changes. We usually close such changes
if made to Rails master as a pull request. Following the same principle
here and reverting.

[ci skip]

For future reference, this is the regex I used: ^\s*#\s*\n(?!\s*#). Replace
with the first match, and voilà! Note that the regex matches a little bit too
much, so you probably want to `git add -i .` and go through every single diff
to check if it actually should be changed.

After a long list of discussion about the performance problem from using varargs and the reason that we can't find a great pair for it, it would be best to remove support for it for now.

It will come back if we can find a good pair for it. For now, Bon Voyage, `#among?`.

There're a lot of places in Rails source code which make a lot of sense to switching to Object#in? or Object#either? instead of using [].include?.

Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>

Booting a new Rails application does not work after this commit [#5359 state:open]

This reverts commit 38a421b3.

Signed-off-by: NJosé Valim <jose.valim@gmail.com>