* Add basic support for enumerating computed styles
The old code was so broken, it was enumerating the style attribute
instead of the computed styles.
So this patch implements a basic enumeration, but not including custom
properties whose computed value is not the guaranteed-invalid value.
* Feedback
For example,
style.cssText = "margin: 1px 2px 3px 4px";
[...style];
will now be
["margin-top", "margin-right", "margin-bottom", "margin-left"]
instead of
["1px", "2px", "3px", "4px"]
