feat(forkJoin): accept promise, resultselector as parameter of forkJoin

[kwonoj]

closes #507

forkJoin now accepts promises as well as result selector, and expanded test coverage.

[kwonoj]

Updated PR. Appreciate take time to review with suggestions, @Blesh

[benlesh]

Looks really good. I found the most minor thing in the world (a let instead of a const). You don't have to fix it if you don't want to, we can just merge the changes. Just thought I'd point it out.

[kwonoj]

You don't have to fix it if you don't want to

I've update it, though I usually try to be strict on code in this case I obviously missed it. Thanks for pointing it out.

[benlesh]

I've update it, though I usually try to be strict on code in this case I obviously missed it. Thanks for pointing it out.

No problem, what I try to do is const everything and then let the compiler or IDE complain at me when I need a let

[benlesh]

Merged with 190f349 ... thanks! Good work!

[staltz]

I hope you don't mind nitpicking, but I noticed 3 spaces for indentation here. Should be 2.

[kwonoj]

I don't mind it at all, actually I also do similar thing when I review with my colleagues :) I'll check codes and apply changes if necessary.