Fix unnecessary display of data frame icon in name column

One more to consider for the preview update. Without it data frames show two icons:

![image](https://f.cloud.github.com/assets/470418/1018399/215fe0a8-0c28-11e3-9d3b-8272fc47a704.png)

Happened when switching `background: none` [which cleared the inherited image] to `background-color: #ffffff`, which did not.