Fixed type of ICustomer.multipass_identifier

[natewaddoups]

I suspect that "multipass_identifier: null" was never the intent...

[natewaddoups]

There's a bigger problem here - the createCustomer mutation expects an object with camelCased properties, but the shopify-api-node library requires an object with snake_cased properties.

Reopening the PR because the createCustomer mutation works with both snake_case and camelCase, so the existing property names are just fine.

[coveralls]

coverage: 100.0%. remained the same
when pulling 2dec9b8 on natewaddoups:master
into fd6d034 on MONEI:master.

[lpinca]

Thank you.