Fixed a bug in `when`-method where an array with only one promise was immediately resolved

If `when` was called with only one promise, the resolver was immediately called:

```javascript
_.when( [ _.Deferred() ] ).then( function () {
    console.log("2");
} );
console.log("1");
```

printed

```javascript
2
1
```

All tests are running, but I haven't added a test for this bug because I'm not sure where to put it.

Could you please also bump the version and publish it?