Fix seeming race condition

**Note: I had to unindent the try/finally block, so you might want to view the diff without whitespace: https://github.com/steiza/txroutes/pull/3/files?w=1**

From what I can tell, it's possible for browsers to request `/favicon.ico` in a way that txroutes will append into the same path in the `getChild` function.  I would see sporadic requests like `self.__path` == "/favicon.ico/the/actual/path"

This simplifies path handling to use the actual `request.path` instead.  Tested locally, sporadic failures stopped happening.