Fixed default coordinates bug in fromHTML

[rustamli]

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:

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.

[diegocr]

Apparently i wrongly assumed nobody would want to use a 0 coordinate... well, thanks.