OCSP Responder doesn't work right for paths other than "/"

In a recent pull request, I modified the OCSP Responder's GET path to expect no "/" at the beginning. That means that if your OCSP URLs start with "/ocsp", you'd want to do something like:

```
http.StripPrefix("/ocsp/", ocspHandler)
```

That works properly for GET, but since POST requests go to `/ocsp` instead of `/ocsp/`, the StripPrefix would return 404 for POSTs. Fortunately, if you OCSP URLs are just `/`, things work fine.

I created this bug, so I'll work on fixing it. Just filing it here to keep track.