(unreleased regression) Retrieve customer_id correctly
In 5.3.2, retrieving the customer_id was: $value = $this->_inputParameters->data->object->customer;.
In master, it's retrieved with getParam(), with return (string) $stripeObject->customer;. However, $stripeObject in getParam() refers to the outer event object, so we need to refer to the internal object (or put this back into getObjectParam().
@mattwire If you prefer another way of fixing this, feel free to close this PR and do it your way.