http(s): allow empty values for header fields

http(s): allow empty values for header fields

It seems completely pointless from a server-POV to sent empty header
fields, so most of them don't do it (simply proven by this limitation
existing since day one) – but it is technically allowed by the RFC as
the surounding whitespaces are optional and Github seems to like sending
"X-Geo-Block-List:\r\n" since recently (bug reports in other http
clients indicate July) at least sometimes as the reporter claims to have
seen it on https only even through it can happen with both.

Closes: 834048