Fixed a bug where Element.style() would not save empty values in IE11

Hi,
I noticed a problem with svg.js, which is when the style() method is called to save an empty value in a style property (so it reverts to its default value), the change is not registered by IE11.

A simple test case :
```javascript
<!DOCTYPE html>
<html>
    <body>
        <h1>Test SVG</h1>
        <div id="lel"></div>
        <script src="svg.js"></script>
        <script>
            var draw = SVG('lel');
            var ellipse = draw.ellipse(100, 100);
            ellipse.hide(); // Works
            ellipse.show(); // On IE11, the ellipse does not reappear
        </script>
    </body>
</html>
```
This works as a test case since ellipse.show() will actually call ellipse.style('display', '')

Note that I did not test the fix in all the browsers supported by svg.js. only with the latest versions of IE, Firefox and Chrome (on Windows of course).