http: disallow empty header names (#1690) In theory, we could be more strict, since there is only a subset of allowed characters: > ``` > tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" > / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~" > / DIGIT / ALPHA > ; any VCHAR, except delimiters > ``` But I'd skip such verification, unless there is a legitimate interest (versus performances). Fixes: #1686