Fixes issue #38: switch_buffer not working properly

To duplicate issue:
```
let g:ctrlp_switch_buffer = 'Et'
```
in .vimrc.

Open new Vim instance, edit file A. Edit file B in new tab. Call :CtrlPBuffer, and select file A. Buffer will open in second tab instead of switching to first tab.

This fixes the bug which is caused by bad type conversion.