Change how DOP is formatted in mergeVars() to match MailChimp's date field format

### Situation
DOP date (date of purchase) could be sent in a wrong format to the MailChimp List when subscription is synced.

### Problem
A User reported that using a different date format than "mm/dd/yyyy", this value will be sent in a wrong way.

### Possible solution - How should it be?
Change how MageMonkey parses this date and forcing it to be "mm/dd/yyyy" matching MailChimp Date field format.