Recognise RxObservable.of and others

[jonakyd]

Hi, Sam
I found that although Listr claims that it supports RxObservable but in fact, it only recognises the plainest RxObervable comes from new Observable(). Neither RxObservable.of() nor RxObservable.combineLatest() and more will be recognised.

I add the instance check base on this SO answer. https://stackoverflow.com/a/41452513/1016407

Please let me know if there is any problem and please kindly accept the PR.

[SamVerschueren]

It seems that the issue, which is linked in the code (sindresorhus/is-observable#1), was resolved. I'm looking into it wheter it's really the case, but if it is, we could use is-observable again. I tested Observable.of with is-observable and it works fine.

So hold on a brief moment until I'm sure that the issue was resolved. Thanks for the PR!

[jonakyd]

Thank you. Please let me know if there is anything I can help.

[SamVerschueren]

If it's possible to test your code in the meantime by using is-observable, that would be great :).

[SamVerschueren]

I just added is-observable to detect Observables. I'm releasing the next version today.