Modal autoFocus does not refocus on last focused element when Modal is closed

[vabruzzo]

I opened an issue previously here: react-bootstrap/react-bootstrap#1851

In short, the element triggering the modal is not being refocused when the modal is closed. It appears that what is happening is that componentWillUpdate is called twice, and in turn, checkForFocus is called twice. So the modal is opened, checkForFocus is called, this.lastFocus is set to the triggering element (good), checkForFocus is called again, and this.lastFocus is set to the modal (assuming that autoFocus is set to true). I don't know why componentWillUpdate is being called twice, but I double checked that my own component is being rendered only once. This issue also affects the modals in the react-bootstrap doc., so I don't think my own code is causing the issue.

A simple fix appears to be to check that both nextProps.show is true (which is the current check) and that this.props.show is false within componentWillUpdate. This prevents the call to checkForFocus occurring when it doesn't need to.

I'll submit a pull request in a few, in the mean time I'm curious to know if I'm just way off track here :)