Strip leading / from OCSP requests

[rolandshoemaker]

In the LE certificate profile we have a trailing / in our OCSP AIA extension, this causes some UAs to blindly contact {ocsp-uri}/{ocsp-request} when generating their GET URL which generates technically malformed requests, e.g. example//request, which ocsp-responder will not properly parse.

Both the previous use of http.DefaultServeMux and the default slash collapsing behavior of nginx hid this from us in the past but after #2748 we started throwing base64 decoding issues since the requests contained leading slashes.

[rolandshoemaker]

Closed by #2778.