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.

Conflicts:
	railties/test/initializer/initialize_i18n_test.rb
	railties/test/initializer/path_test.rb

Fix the documentation for root :to. It should use a fully qualified controller#action syntax (Closes #3606)

Fix ActionMailer. The fact that ActionMailer::Base does not inherit from AbstractController::Base is either a bug or we need to re-evaluate the requirements of the mixins.

Turn filter_parameter_logging on by default for password and password_confirmation and remove contentless comments

AC::Head now doesn't have an unfulfilled Rendering dependency, and instead works just fine standalone (which means that ConditionalGet also doesn't have a Rendering dependency)

Added ActionDispatch::Request#authorization to access the http authentication header regardless of its proxy hiding [DHH]