Patch to use this module with the dev version of Ubercart Discount Coupons (new variable valid_from in the database)

Issue Summary

Hi,

The dev version of the module Ubercart Discount Coupons has added the possibility to define a valid_from date for the coupons.

This patch allows uc_invite_discount to correctly insert in the table uc_coupons this date. Note that it inserts the time when the coupon is created.

Hope this helps :)