groupJoin throws exception if outer sequence contains elements without inner sequence

[bman654]

It is supposed to work as a left outer join, but instead it throws an error if the inner sequence has no matching elements to any of the outer elements.

This will throw an error:

Ix.Enumerable.fromArray([1])
    .groupJoin(
        Ix.Enumerable.empty(),
        function (v) { return v; },
        function (v) { return v; },
        function (o,i) { return o; })
    .toArray();

It should return [1].

[elmerbulthuis]

fixed it in #14