Update the attachment's timestamp if the original file got reprocessed

Closes #170

lib/paperclip/attachment.rb

@@ -263,6 +263,7 @@ module Paperclip
         new_original.rewind
 
         @queued_for_write = { :original => new_original }
+        instance_write(:updated_at, Time.now)
         post_process(*style_args)
 
         old_original.close if old_original.respond_to?(:close)

test/integration_test.rb

@@ -58,6 +58,7 @@ class IntegrationTest < Test::Unit::TestCase
           has_attached_file :avatar, :styles => { :thumb => "150x25#", :dynamic => lambda { |a| '50x50#' } }
         end
         @d2 = Dummy.find(@dummy.id)
+        @original_timestamp = @d2.avatar_updated_at
         @d2.avatar.reprocess!
         @d2.save
       end
@@ -66,6 +67,10 @@ class IntegrationTest < Test::Unit::TestCase
         assert_match /\b150x25\b/, `identify "#{@dummy.avatar.path(:thumb)}"`
         assert_match /\b50x50\b/, `identify "#{@dummy.avatar.path(:dynamic)}"`
       end
+
+      should "change the timestamp" do
+        assert_not_equal @original_timestamp, @d2.avatar_updated_at
+      end
     end
   end