Don't include customer id unless it is numeric.

Similar to #703, we were getting back errors when sending an email address for the customer ID. After contacting Auth.net they have informed us that there is a filter on UK based merchant accounts that doesn't allow ID's that are either email addresses or phone numbers.

I'm not entirely satisfied with this solution - although limiting the customer ID to a numeric value would in theory fix the issue, it cuts out other valid strings. I'm not sure we can actually do any better without them documenting the exact regex of what is invalid.

@jduff @odorcicd