Accessing `then` on a `function` or `object` throws within the `getThen` helper

Here we try to get the then method from an undefined thing called promise.

https://github.com/stefanpenner/es6-promise/blob/314e4831d5a0a85edcb084444ce089c16afdcbe2/lib/es6-promise/-internal.js#L33

I think it should look like Promise.then, since Promise is defined.

I've seen this in IE11 (no Promise there) and we always run into the catch branch. I'v thought that this is not its intention.