Lenient HTTP version parsing

Invalid HTTP version (lenient)

HTTP/5.6 200 OK

off=0 message begin
off=0 len=4 span[protocol]="HTTP"
off=4 protocol complete
off=5 len=3 span[version]="5.6"
off=8 version complete
off=13 len=2 span[status]="OK"
off=17 status complete
off=19 headers complete status=200 v=5/6 flags=0 content_length=0