The __pairs metamethod was introduced in 5.2, so instead of using pairs()
on http.headers values, use its :each() method — which is what the __pairs
metamethod calls anyway.
When receiving a HTTP response without a statusline, "nil" is returned as
reponse headers and status line values, but the response code will still
be "200 Success", which is quite not right as there wasn't ever a response
status code received. This patch adds a check for a status line, and returns
with an early error if needed.
This condition can be triggered by trying to speak plain HTTP to a
HTTPS-enabled server, e.g. using http://matrix.org:8448 instead of
https://matrix.org:8448 as homeserver URL.
