Merge pull request #5467 from bogdan/initialize_callbacks

ActiveRecord::Core#initialize: improve performance

activerecord/lib/active_record/core.rb

@@ -176,7 +176,7 @@ def initialize(attributes = nil, options = {})
       assign_attributes(attributes, options) if attributes
 
       yield self if block_given?
-      run_callbacks :initialize
+      run_callbacks :initialize if _initialize_callbacks.any?
     end
 
     # Initialize an empty model object from +coder+. +coder+ must contain