Revert "Reduce memory by not duplicating an argument"

This reverts commit 0ad9c66.

Author: joshuacronemeyer

lib/timecop/time_stack_item.rb

@@ -56,7 +56,7 @@ def scaling_factor
 
     def time(time_klass = Time) #:nodoc:
       if @time.respond_to?(:in_time_zone)
-        time = time_klass.at(@time.localtime)
+        time = time_klass.at(@time.dup.localtime)
       else
         time = time_klass.at(@time)
       end