Interpret any string in options as an API Key instead of using a regex

[rattrayalex-stripe]

r? @ob-stripe
cc @stripe/api-libraries @jlomas-stripe @paulasjes-stripe @remi-stripe

isAuthKey relies on a regex which assumes a fixed length of a secret key token. This assumption recently broke.

Looking at the code, passing a string which does not fit the regex currently noops completely, so I chose to simply check whether it is a non-empty string and if so, assume it is an auth key.

Note that you could maybe interpret this to be a breaking change, because you could have previously written this:

stripe.customers.retrieve('cus_123', 'garbage');

which would have been equivalent to stripe.customers.retrieve('cus_123') and worked fine, but with this change would fail with a bad secret key.

I think that's fine, but not sure and want to check with others.

Relatedly, I moved isAuthKey from utils but that didn't seem to be publicly exported so I think that's fine.

[ob-stripe]

lgtm