fix: unsubscribe better. Fixes error when host uses drag / mouse events

[ronlawrence3]

This fixes a failure when the host element has handlers for drag events, and only unsubscribes the subscriptions if they were subscribed. I was getting this during drag events:

GroupElementComponent.html:10 ERROR TypeError: Cannot read property 'unsubscribe' of undefined
    at ResizableDirective.push../node_modules/angular-resizable-element/fesm5/angular-resizable-element.js.ResizableDirective.ngOnDestroy (angular-resizable-element.js:580)
    at callProviderLifecycles (core.js:9268)

I'm not sure why the ngOnDestroy was called without the subscriptions being there, but this fixes the error.

[ronlawrence3]

Found this, which seems to back up what I'm seeing.
https://www.bennadel.com/blog/3356-ngoninit-may-not-get-called-before-ngondestroy-is-called-in-angular-4-4-6.htm

[mattlewis92]

Ah, I've seen this kind of error before as well, makes sense. Travis is having some issues at the moment, will merge once it's backup

[codecov-io]

Codecov Report

Merging #82 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #82      +/-   ##
=========================================
- Coverage   94.83%   94.8%   -0.04%     
=========================================
  Files           4       4              
  Lines         310     308       -2     
  Branches       48      48              
=========================================
- Hits          294     292       -2     
  Misses          8       8              
  Partials        8       8

Impacted Files	Coverage Δ
src/resizable.directive.ts	95.13% <100%> (-0.04%)	⬇️

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[mattlewis92]

Released as 3.2.2, thanks for your contribution! 😄

[ronlawrence3]

👍 Thank you!