Handle case where resizeTarget is not an event listener

[brianium]

@FezVrasta many thanks for this library. I've been using it with popper.js and it is 💯

I ran into an issue recently with Safari throwing errors when using this component inside of a react portal.

It seems that in this browser React stubs in a window for contentDocument.defaultView that is not actually an event listener. So when ResizeAware unmounts in Safari - it throws an error complaining that removeEventListener is not a function.

I included a simple fix that solves my issues, and might just make things more robust in general?

Let me know if I can do anything to help get this merged 😄

Many thanks again for the work you do on these libs!

[FezVrasta]

Thanks! May you add a little comment there to explain what's going on/why it's needed, for future reference?

[brianium]

Sure thing!

[brianium]

done.

[FezVrasta]

published as 2.7.1

[brianium]

Thank you!