Angular 8 compilation issues

[MichalGwozdz]

Hi,
After updating my app to Angular 8 I received some issues from your library:

and

Looks like it's very simple to fix (I checked how it is done in original xlsx lib).
I created pull request based by solution from original xlsx library:
#136

[AliSawari]

@MichalGwozdz
will this be merged soon?