jsnx.allPairsDijkstraPath fails with complex node types

The following code will work when nodes are primitive objects, but not when complex objects.

``` javascript
function test(p) {
var G = new jsnx.Graph();
G.addStar(p, {weight: 1.0});
return jsnx.allPairsDijkstraPath(G)
}

test([1,2,3,4]); // works
test([{name: "cat"},{name: "dog"},{name: "mouse"}]); // fails
test([[0,0],[0,1],[1,0]]); // fails
```

The error message is

> Uncaught TypeError: Cannot read property 'toString' of undefined(…)
> n @ jsnetworkx.js:1
> d @ jsnetworkx.js:3
> s @ jsnetworkx.js:3
> b @ jsnetworkx.js:3

Note, that the code will work when allPairsDijkstraPath is replaced with allPairsShortestPath.