Fix cookie default path

* Fix issue #50
According to [rfc2109](http://www.w3.org/Protocols/rfc2109/rfc2109) 
```
   Path   Defaults to the path of the request URL that generated the
             Set-Cookie response, up to, but not including, the right-most /.
```
That is to say if your request url is `/cookie/default-path` the default path for the newly generated cookie is `/cookie`.  so when send request to `/cookie/default-path/`, the cookie's path will be `/cookie/default-path` and that's what the test case is for.

* fix a bug in fake_app