HOTFIX: Fixed a Custom Fields invalid cast exception occurring on the Receipt Page

Fixed an invalid cast on custom fields section for the receipt page. 

Fixes this error:

Base Exception: System.InvalidCastException: Specified cast is not valid.
at AmeriCommerce.checkout.receipt.rpCustomFields_ItemDataBound(Object sender, RepeaterItemEventArgs e) in f:\Build\AmeriCommerce\Production\2010.5\Store\receipt.aspx.cs:line 243