fix behavior when after_initialize is defined and a block is passed to Base.create

The previous behavior was unintentional, and some people was relying on it. Now the dynamic finder will always expecting the number of arguments to be equal or greater (so you can still pass the options to it.)

So if you were doing this and expecting the second argument to be nil:

    User.find_by_username_and_group("sikachu")

You'll now get `ArgumentError: wrong number of arguments (1 for 2).` You'll then have to do this:

    User.find_by_username_and_group("sikachu", nil)

Cache key was incorrectly using timezone-dependent record#updated_at when it should be using a timezone-independent value to generate the cache key

Minor refactoring to cache_key timezone test

Closes #2059

Adds a test to validate the format of the cache_key for nil and present updated_at values

Correctly handles updated_at == nil

Define to_ary on ActiveRecord::Base to return nil. Improve performance of Array#flatten under 1.9, see comment for details.

Perf fix - Use an instance variable instead of a class_attribute. Thanks @josevalim and @jhawthorn for the prompting.

ActiveSupport::Dependencies now raises NameError if it finds an existing
constant in load_missing_constant. This better reflects the nature of
the error which is usually caused by calling constantize on a nested constant.

Closes #1423

Addresses an inconsistency in the ActiveRecord::Base.method_missing handling of dynamic finder methods and the passing of the &block parameter for :find_by_attributes.

Add ActiveRecord::attribute_names to retrieve a list of attribute names. This method will also return an empty array on an abstract class or a model that the table doesn't exists.