Fix _default_px function.

[jdfreder]

No description provided.

[jasongrout]

When would value be a number? I thought all of the traits were unicode. Also, even if we have a number, shouldn't we convert it to a string and append 'px'?

[jdfreder]

I'm not sure when it is a number, but that's and other non-(string|null|undefined) values are the only failing case I noticed. Maybe one/more of the widgets are using default_px on non-unicode traits when they shouldn't be.

I pass numbers through, allowing jQuery to decided what the default unit should be. When we ween the widgets from jQuery, we may want to make this explicit by returning String(number) + 'px'.