Tests: Revert testCreateSubscriberWithInvalidCustomFields

[n7studios]

Summary

Creating a subscriber with invalid custom fields via the API appears to no longer return a HTTP error code, resulting in the testCreateSubscriberWithInvalidCustomFields test assertions failing.

This PR resolves by reverting #118, which originally changed the testCreateSubscriberWithInvalidCustomFields assertions due to this PR.

Testing

Existing tests pass.

Checklist

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