Incorrectly identifying promises

I've encountered an issue in the latest (7.1.0) release relating to commit 492d53e705acb6c

The issue is that isPromise makes a huge assumption that any object with a then method is a Promise. If the object is not a Promise, then will be called which may not be desirable.

This can be demonstrated very simply

Immutable({then:function() { console.log("should not be called") }});

I encountered this when using superagent which provides a thenable interface but is not technically a Promise. I suspect there may well be other libraries with a similar approach.