CRM_Stripe_Customer::find() is only called from CRM/Core/Payment/Stripe.php
The returned value is saved as $stripeCustomerId and then passed to the Stripe API to retrieve the customer record.
Clearly the calling code is expecting to receive a Stripe customer_id such as cus_123456 rather than the id column from the civicrm_stripe_customer table.
This PR corrects CRM_Stripe_Customer::find() to return the customer_id and brings it in line with changes already in 6.8