[TIMOB-6055] iOS: Serif generic font selector not supported on iOS

On Android, the fontFamily can use the generic specifiers 'serif', 'sans-serif', or 'monospace' to select the default system serif, sans-serif, or monospaced font. These also work in CSS, so web developers are used to using them. The 'monospace' selector is implemented on iOS, and the 'sans-serif' selector works by accident (since the default font is sans-serif). However, 'serif' is not recognized. Compare the results of the following code snippet on iOS and Android:

function makeLabel(labelText, labelFont) 
{
	return Titanium.UI.createLabel({
	top: 20,
	text:labelText,
	font:labelFont,
	width:'auto',
	height:'auto'
	});
}

win2.add(makeLabel('Sans-Serif', { fontFamily:'sans-serif', fontSize: 30} ));
win2.add(makeLabel('Serif',      { fontFamily:'serif', fontSize: 30} ));
win2.add(makeLabel('Monospace',  { fontFamily:'monospace', fontSize:30}  ));