lavr: Do not change the sample format for mono audio

This treats mono as planar internally within libavresample rather
than changing the sample format.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
http: Refactor http_open_cnx

Split return value handling from the actual opening.

Incidentally fixes the https -> http redirect issue reported by
Compn on behalf of rcombs.

CC: libav-stable@libav.org