Event Handlers Don't Update

[gonzofish]

I'm using 2.3.0 and noticed that the code doesn't account for an event handler prop changing. So, for instance, if I have a component with:

const MyComponent = ({ someCallback }) => (
   <Plot
     data={data}
     onRelayout={someCallback}
   />
);

The someCallback that is called is the first one. Here's a Codepen reproducing:

https://codepen.io/gonzofish/pen/OevOjx

To see it in action, just click autoscale on the chart and you'll see the message change to "Callback 1 Reporting In". If you change the callback by clicking "Change to 2" and then clicking autoscale again, the message doesn't change, despite the callback being changed.

I believe the issues stems from syncEventHandlers only account for two modes: (1) the handler isn't already set but has been provided as a prop and (2) the handler was set but the prop is not longer there. That starts at:

https://github.com/plotly/react-plotly.js/blob/master/src/factory.js#L212