[TIMOB-14565] iOS: "font" property does not return to default when set to null or {}

problem

When setting the *font* property of a Ti.UI.Label to *null* or *{}*, the font of the Label does not return to the default font which is used when no font property is defined. It instead is set to a font slightly smaller than the default font. This is best seen by running the test case below. This is an issue for Alloy as it relies on the ability to "reset" Titanium proxy properties in order to remove/reset style classes at runtime. In its current state, this could potentially create unexpected visual results on iOS.

expected

When I set the the *font* property of a Ti.UI.Label to *null* or *{}*, the font of the Label should return to its default state. This is what occurs on Android and Mobileweb.

test case

app.js

var win = Ti.UI.createWindow({
	backgroundColor: '#fff',
	modal: false,
	exitOnClose: true
});
var label = Ti.UI.createLabel({
	text: 'just a test label'
});

// Clicking the label will show that on iOS a slightly smaller font is used when
// the label is re-rendered with font=null. All other platforms use the same default
// font.
label.addEventListener('click', function(e) {
	label.font = null;
});

win.add(label);
win.open();