Modify Promise check, Closes #20

[aulisius]

Based on discussion in #20 and googling around, I found this

https://stackoverflow.com/questions/27746304/how-do-i-tell-if-an-object-is-a-promise#27746324

It seems checking for the existence of a then on the prototype/instance is the most consistent solution.

I'm still thinking if there is a way to improve on this.

@andyfleming @joepie91 Any thoughts?

[coveralls]

Coverage remained the same at 100.0% when pulling 2efc2f7 on aulisius:patch-1 into 078528e on andyfleming:master.

[joepie91]

LGTM :)

[bobvanmanen]

Another use case for this is when async-listener is used. It overrides the global Promise into a WrappedPromise. If the interval function is an async function, the check will fail.

I wonder though, wouldn't it be better to just use this to ensure the returned value is always a Promise:

const returnVal = Promise.resolve(func(currentIteration, stop))

[andyfleming]

@bobvanmanen — I'm not sure about long-term, but initially I'm hesitant to make the assumption that non-promise values should just be resolved.

[andyfleming]

Thanks for your contribution @aulisius!