parseCustomerUpdate accepts { data } with a subset of the seven fields the
shop's checkout sends: email, acceptedTermsAndConditionsAt, invoiceAddress,
deliveryAddress, invoiceAddressStructured, deliveryAddressStructured and
delivery. Any other key, at the top level, in data or in a structured
address, rejects the whole body, and the errors name the fields without
repeating their values.
The terms are recorded at the server's time whenever the client sends a
true value: the invoice is dated by it.
Refs libretech/mp#71
