Change PaymentModel#date type to datetime_utc

By changing the type of `PaymentModel#date` to `datetime_utc`, we can
pass timestamps with UTC offsets to Xero's payments API endpoint, and
let Xero parse the dates in the proper timezone for the Organisation in
context.

lib/xeroizer/models/payment.rb

@@ -13,7 +13,7 @@ class Payment < Base
       set_primary_key :payment_id
 
       guid          :payment_id
-      date          :date
+      datetime_utc  :date
       decimal       :amount
       decimal       :currency_rate
       string        :payment_type