Update circle.class.js

svg
```xml
<?xml version="1.0" encoding="utf-8"?>
<!-- Generator: Adobe Illustrator 16.0.0, SVG Export Plug-In . SVG Version: 6.00 Build 0)  -->
<svg version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px"
	 width="600" height="400" viewBox="0 0 600 400" enable-background="new 0 0 56.667 60.667" xml:space="preserve" >
<circle opacity="0.2" fill="#900" cx="50" cy="70" r="44"  />
</svg>
```
before fix
![image](https://cloud.githubusercontent.com/assets/1194048/3707723/c4e0c6f2-1438-11e4-8525-2e203fb6774c.png)
after fix
![image](https://cloud.githubusercontent.com/assets/1194048/3707701/82f73be0-1438-11e4-8a6c-7fbbd19dde7c.png)