Fixed white background problem (again)

In regards to #115 and #116:

I'm sorry I messed it up. I hadn't tested my fix properly.

So apparently if the `background-color` style is not set in the CSS the computed value in Phantom/Chrome isn't falsy but `rgba(0, 0, 0, 0)` (transparent). So the check in the `if` statement would never catch it and still leave the background color unset.

This patch now checks for both falsy (never know) and `rgba(0, 0, 0, 0)`.

Here are some popular sites that don't have a background color:

```
linkedin.com
microsoft.com
msn.com
taobao.com
weibo.com
yahoo.co.jp
baidu.com
```

They now get a white background while sites that do have a color remain untouched.