RFC 1738 says the slash is optional:
; HTTP httpurl = "http://" hostport [ "/" hpath [ "?" search ]] hpath = hsegment *[ "/" hsegment ]
RFC 3986 talks about normalization, and it recommends adding a slash:
In general, a URI that uses the generic syntax for authority with an empty path should be normalized to a path of "/".
It also makes the slash after the domain name optional when serializing an URL:
An absolute URL must be a scheme, followed by ":", followed by either a scheme-relative URL [...]
[...]
A scheme-relative URL must be "//", optionally followed by userinfo and "@", followed by a host, optionally followed by ":" and a port, optionally followed by an absolute-path-relative URL.
[...]
An absolute-path-relative URL must be "/", followed by a path-relative URL that does not start with "/".