Fix duck test to match Zone interface

[justinbhopper]

Fixes #1426

normalizeZone uses offset on a Zone for the duck test, but incorrectly assumes it is a property that returns a number when in fact it is a function/method on Zone.

The duck test is particularly important for scenarios where package dependencies brought in their own copy of luxon (perhaps due to version mismatches), causing the instance of check to fail. The duck test helps ensure compatibility with other instances of luxon modules.

[linux-foundation-easycla[bot]]

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• :white_check_mark: login: justinbhopper / name: Justin Hopper (0acddaa2c6adf75ac37d35fd0d43990649568ada)

[icambron]

@justinbhopper thanks