Allow nil values in user update payload

Gabe Taylor Russrequested to merge
gabe9250201/iterable:allow-nil-when-updating-user into master

Hi @mtchavez! First of all, thank you for creating and maintaining the Iterable API client.

This change updates the type signature for the User update payload to allow for nil values. We pass nil to unset existing values in Iterable where required.

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