htmlizer entry point does wrong thing with cased inputs

From @agmalis

> I note that the URL
> 
> https://datatracker.ietf.org/doc/html/rfc7209
> 
> correctly displays RFC 7209.
> 
> However, the URL
> 
> https://datatracker.ietf.org/doc/html/RFC7209
> 
> actually displays draft-ietf-l2vpn-evpn-req-07.txt .
> 
> Cheers,
> Andy