iOS: Animate the Picker Off-Screen

[joncursi]

The Picker will animate onto the screen when invoked with a smooth transition, however, when the modal is dismissed through either Cancel / Confirm / clicking on the backdrop, it instantly flashes / disappears off-screen.

It should instead animate off the screen with the exact but inverted animation that was used to bring it onto the screen.

[kathawthorne]

I just opened a PR to fix this; hopefully it will make it in :)