fix collapsible content top and bottom spacing

Before:
![image](https://f.cloud.github.com/assets/677704/727200/f0cebd1e-e176-11e2-84dd-fda4da009602.png)
After:
![image](https://f.cloud.github.com/assets/677704/727202/07c18470-e177-11e2-8aa5-01246d289f32.png)