Fix for empty elements that are not self closed

In a situation where the xml to parse contains empty elements that are not self closed, swiz behaves strangely.
```
eg. 
<root><element></element></root>
```
Swiz would produce:
```javascript
{
element:{ element: null}
}
```

This fix would create an object that looks like the following:

```javascript
{
element: null
}
```

Provided you set stripNulls:false.

I also created a test for this case that passes.  I am not 100% sure if more tests for other cases are needed.