Corrected the referral fields on Retrieve customers

Two loyalty fields in the Retrieve customers response (with expand=loyalty) are now described correctly:

• referred_by_customer: the email address of the customer whose referral link referred this customer, or null if the customer wasn't referred.
• is_referred_by_other_customer: currently returns the opposite of its name. It's false for customers who were referred and true for customers who weren't. Until this is fixed, check whether referred_by_customer is set to tell whether a customer was referred.