Unhandled Promise rejections from instanceof check

[metrip]

The handleRequest function checks for result.then instanceof Function to determine if a Promise result is returned from the handler, in https://github.com/ctimmerm/axios-mock-adapter/blob/master/src/handle_request.js#L52

This does not seem to be correct, as this is not true for returned rejected promises in all environments; in my case Node.js 8.10. I'm using adapter.onGet(...).timeout() and I see:

This leads to the flow going to utils.settle instead of result.then, the promise not getting handled and erroring, and the original error getting lost, which is bad.

Possibly related: #122

I believe adding the typeof check would do the intended functionality correctly here?

[ctimmerm]

Do you think you could write a failing test case for it?

[metrip]

On further inspection, it is not a feature of Node, but somehow specific to the VM that Jest sets up - Promise objects look a little different there, but I can't really tell why. Could be occurring in VM's created by other test frameworks, but I haven't had a look at them.

Writing test cases seems difficult as I can't find a good way to simulate functions like this.

However, as the intent here seems to be "if result.then is callable, settle result like a Promise", so the solution should be simple somehow.