onExit doesn't fire unless viewportSpy is enabled

[XuluWarrior]

#270 added the viewportSpy option which means that if it is disabled, after onEnter, stopWatching is called.

This makes sense if you're only considering onEnter but if onExit is defined, then it is counterintuitive that it never gets called unless viewportSpy is set to true.

It took me a while to realise why {{in-viewport onEnter=this.onEnter onExit=this.onExit}} only called my onEnter handler.

I would propose a change to the check on

ember-in-viewport/addon/modifiers/in-viewport.js

Line 45 in e052a89

if (!this.options.viewportSpy) {

to

if (!this.options.viewportSpy && !this.args.named.onExit) {