If isEditable is enabled Background is not properly set

Currently if you attempt to change the background color of a combobox that has the property `isEditable="true" ` only the background of the dropdown arrow will change.

![Bug](https://cloud.githubusercontent.com/assets/5459225/4566653/d9476cf4-4f2a-11e4-9886-d03b8718f9a2.png)

This commit makes sure the property is correctly set.