Fixed default coordinates bug in fromHTML

It seems there was no way to set starting coordinates to **0** using _fromHTML_ method,

When you specified x = 0 or y = 0, they defaulted to 4.

This was caused by following snippet from **jspdf.plugin.from_html.js**:

```javascript
jsPDFAPI.fromHTML = function (HTML, x, y, settings, callback, margins) {
	// ...
	return process(this, HTML, x || 4, y || 4, settings, callback);
}
```

Now they default to **4**, only in case when their values are _undefined_.