Log a warning when Content-Type is not correct

When a feed is served with a wrong Content-Type, log a warning and not an error.
This prevents a "processing error" message from being displayed at the default
log level, which is confusing for a innocuous "error" like this.

See https://bugs.debian.org/760963.

Signed-off-by: Etienne Millon <me@emillon.org>