Fix callback for nested signIn or addScopes

[AntonKozlovskyi]

Fixes #24

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[AntonKozlovskyi]

@googlebot I signed it!

[petea]

Thanks, Anton! Could you also apply this fix to the two other locations in GIDSignIn.m where we set "_currentOptions = nil" after executing the callback block?

[AntonKozlovskyi]

done

[tgun1370]

AntonKozlovskyi:Fix_missing_completion_handler_on_add_scopes