Fix issue #2

Replacing this:

```javascript
if (a[prop] !== b[prop]) {
    c[prop] = [a[prop], b[prop]];
}
```
with

```javascript
if(a===undefined) a = {};
if(b===undefined) b = {};
if (a[prop] !== b[prop]) {
    c[prop] = [a[prop], b[prop]];
}
```
fixes the issue by making sure the object is initialized.