Fix customer/subscription retrieval when stored with integer id

When creating a customer or a subscription with integer as id, it cannot be retrieved later.
The requested id is received as string over HTTP, the attributes are stored under an integer key in the registry.

This PR fixes the issue by normalizing the registry key to a string upon creation.

As far as I know only Customer and Subscription allow custom ids to be set, skimmed through the Braintree documentation quickly to verify.

The solution is more a quick one, not that dirty, but the normalization logic would better reside in the registry.
I split the solution into specs and fix, if you only want to pick the specs and apply a different solution.

Cheers!