This reverts commit 1c02b988.

This reverts commit 93b1ca8b.

Thanks to @jeremy to teach me this one.

This reverts commit 4e9f53f9, reversing
changes made to 6b802cdb.

Revert "Don't use tap in this case."

This reverts commit 454d820b.

Reason: Is not a good idea to add options to this method since we can do
the same thing using method composition.

    Person.validators_on(:name).select { |v| v.kind == :presence }

Also it avoids to change the method again to add more options.

The use of tap in this case is very confusing since we are mutating the
return value inside the block

This will filter out the validators on a particular attribute based on
its kind.

This commit allows a user to do something like:
    before_validation :do_stuff, :on => [ :create, :update ]
    after_validation :do_more, :on => [ :create, :update ]

attr_accessible and attr_protected raise an exception pointing to use plugin or new protection model

This will be moved out to protected_attributes gem

The new option allows any Ruby namespace to be registered and set
up for eager load. We are effectively exposing the structure existing
in Rails since v3.0 for all developers in order to make their applications
thread-safe and CoW friendly.

- Enable propagation of :skip_types, :dasherize and :camelize on included models by default
- Adding the option to override this propagation on a per-include basis (:include => { :model => { :dasherize => false } }
- Enough tests to prove it works
- Updated activemodel CHANGELOG.md

Squashed my commits

Under a "private" call, class methods are not real private methods,
they're public just like any other method.

Make model_name_from_record_or_class a private class method, and nodoc it.

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.

errors_options is calculated for every *CHECKS* but it could be
calculated one time  because options and RESERVED_OPTIONS
not change during this cycle

On reflection, it seems like a bit of a weird method to have on
ActiveRecord::Base, and it shouldn't be needed most of the time anyway.

Link to ActiveRecord::Serialization#to_xml for more details