Use path_info instead of path to match /_ping

[path](http://rack.rubyforge.org/doc/Rack/Request.html#method-i-path) includes [script_name](http://rack.rubyforge.org/doc/Rack/Request.html#method-i-script_name) which means that if an app's root is `example.com/app`, `example.com/app/_ping` won't work by default. Checking against [path_info](http://rack.rubyforge.org/doc/Rack/Request.html#method-i-path_info) excludes the script name which means that it will check `/_ping` against the app root instead of the domain.

So if the app is hosted at `app.example.com`, `app.example.com/_ping` will work by default, and if the app is hosted at `example.com/app`, `example.com/app/_ping` will work as well.