Bug: custom element properties can't accept functions

[josepharhar]

This was raised here: #11347 (comment)

I will open a PR to address this shortly. Please assign this bug to me.

React version: experimental

Steps To Reproduce

1. Create a custom element with a property setter
2. Assign a function to that property

Link to code example: https://jsfiddle.net/jarhar/df4k156b/

The current behavior

The function does not get passed to the custom element setter

The expected behavior

The function gets passed to the custom element setter