ActionView::Helpers::FormHelper refactoring

Action controller refactor - remove Compatibility module

AS::Callbacks: Refactor :per_key option

Fix bug where reset_counters resets the wrong counter cache.

ActionView::Helpers

[Proposal] We should deprecate the RAILS_CACHE constant.

Add option to configure Sprockets logger

This should be set globally as a configuration, using
`config.action_dispatch.default_charset` instead

This seems to be required only when calling render :partial with an
empty collection from a controller. This call happens to return no
content, letting the response body empty, which means to Rails that it
should go on and try to find a template to render based on the current
action name, thus failing hard.

Although tests keep all green, we need to check a better way to fix
this.

Do not create a method_missing method to handle not found actions, use
the action_missing method provided by Rails instead.

Options :nothing and :text => nil should be handled by
ActionController::Rendering instead.

Based on 50d23bc2.

  render :template => "/foo/bar"

This is already being set by Rails configuration.

Validates method should not change options argument

when validating a record, if a validation context is used, use the same context when validating related records 

use thread variable in TaggedLogging

Wrong behavior of ActiveModel::Errors#dup is causing regressions on Rails master

Since ActiveModel::Errors instance keeps all error messages as hash
we should duplicate this object as well.

Previously ActiveModel::Errors was a subclass of ActiveSupport::OrderedHash,
which results in different behavior on `dup`, this may result in regression for
people relying on it.

E.G.:

```ruby
class Parent < ActiveRecord::Base
  has_one :child
  validates_presence_of :name, :on => "custom_context"
  validates_associated :child
end

class Child < ActiveRecord::Base
  belongs_to :parent
  validates_presence_of :name, :on => "custom_context"
end

p = Parent.new(:name => "Montoto", :child => Child.new)
p.valid?(:custom_context) # => Returns true, even though the child is not valid under the same context.
```

If a model belongs_to two associations with the same class, then reset_counters
will reset the wrong counter cache.

Finding the right reflection should use the foreign_key instead, which should
be unique.