Mainly around `nil`

[ci skip]

Closes #11209

[Roque Pinel & Steven Yang]

The documentation on `:on` for validations was inconsistent, and most
only referenced the `:create` and `:update` contexts. I fixed those to
be consistent with the documentation on `AM::Validations.validates`,
which seemed to have the best docs.

[ci skip]

Now that Validator #setup is called from the initializer, we need a
reference to the model's class to be passed in to allow the validators
to continue functioning when used at the instance level.

Closes #14134.

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

There's no need to create two extra hashes with options.merge(another_hash),
with the goal of setting only one value, so lets just set it.

Also refactor validates_each to use _merge_attributes, like other
validates_* helpers do.

In order to deliver debug information to dev team
instead of display error message to end user
Implemented strict validation concept
that suppose to define validation that always raise exception when fails

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

class Person < ActiveRecord::Base
  include MyValidators

  validates :name, :presence => true, :uniqueness => true, :length => { :maximum => 100 }
  validates :email, :presence => true, :email => true
end

[#3058 status:resolved]
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

Move validations in ActiveModel to validators, however all validatity checks are still in the class method.

Validator is simply sent to validate method. However, the API needs to change, so validate accepts a record.

Changed ActiveRecord to use new callbacks and speed up observers by only notifying events that are actually being consumed.
Signed-off-by: NJoshua Peek <josh@joshpeek.com>

[#2630 state:committed]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>