Newly instantiated STI records with no changes are marked as dirty, PR for test and fix

Example:

    class Parrot < ActiveRecord::Base
    end

    class LiveParrot < Parrot
    end

    # Before:
    Parrot.new.dirty? # => false
    LiveParrot.new.dirty? # => true

    # After:
    Parrot.new.dirty? # => false
    LiveParrot.new.dirty? # => false