…eing copied.

Change in class method Plan.deep_copy:
  - Firstly, on duplicating the Plan we set the plan identifier to nil and save.
  - Then we fill the identifier variable with the Plan id that was
      regenerated when the duplicate copy was save in the previous.
  - We then persist this change by saving the Plan again.

…eing 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