code: public AddOn updateAddOn(final String planCode, final String addOnCode, final AddOn addOn) {
          return doPUT(Plan.PLANS_RESOURCE +
                  "/" +
                  planCode +
                  AddOn.ADDONS_RESOURCE +
                  "/" +
                  addOnCode,
                  addOn,
                  AddOn.class);
      }
docs: Updates an {@link AddOn} for a Plan
  <p>
  
  @param planCode  The {@link Plan} object.
  @param addOnCode The {@link AddOn} object to update.
  @param addOn The updated {@link AddOn} data.
  
  @return the updated {@link AddOn} object.