"Returns a Promise" check is incorrect

[joepie91]

This library currently checks whether the callback returned a Promise by doing returnVal instanceof Promise, but this is incorrect; it only considers the global ES6 Promise implementation in the library's scope/realm, and not any other Promise implementations (including cross-realm Promises, non-global polyfills, and other Promises/A+ compliant Promise implementations such as Bluebird).

The correct check, as per the Promises/A+ specification, would be more something like:

returnVal != null && typeof returnVal.then === "function"

[andyfleming]

Good catch @joepie91. We'll see if we can get this addressed. 👍