Recognize current tab with String#starts_with? instead of regexp

If plugin has sub-routes same as default tab names, active tab is recognized incorrectly.

![Screenshot](http://f.cl.ly/items/3m0j3y3o1d0X033J1447/Screen%20Shot%202013-05-07%20at%206.58.25%20PM.png)

This PR fixes that behavior.