Fix for bug in Plan duplication results in original plan identifier being copied.

Changes (suggested by @aaronskiba):
  - removed an unnecessary line
      plan_copy.identifier = nil
  - cast to string the integer-valued plan id
    plan_copy.identifier = plan_copy.id.to_s

Author: John Pinto

app/models/plan.rb

@@ -254,11 +254,9 @@ def self.deep_copy(plan)
     plan_copy = plan.dup
     plan_copy.title = "Copy of #{plan.title}"
     plan_copy.feedback_requested = false
-    # Don't copy the identifier as it will be regenerated
-    plan_copy.identifier = nil
     plan_copy.save!
     # Copy newly generated Id to the identifier
-    plan_copy.identifier = plan_copy.id
+    plan_copy.identifier = plan_copy.id.to_s
     plan.save!
     plan.answers.each do |answer|
       answer_copy = Answer.deep_copy(answer)