:path ought to be HTTP's absolute-path, not RFC3986's path-absolute

[wtarreau]

There is currently an incompatibility between HTTP/1 and HTTP semantics definition of absolute-path and H2's use of RFC3986's path-absolute, which prevents paths from starting with //. I described it there but can paste it here if required for ease ofreading:

https://lists.w3.org/Archives/Public/ietf-http-wg/2021JulSep/0245.html

My feeling is that it was an unintentional mistake introduced between http2 drafts 4 and 5 given that it breaks sites and doesn't even look respected by H2 agents (i.e. curl, apache, varnish, firefox happily process // in h2).