Handle only "200 OK" response as valid SSDP response

According to the spec, the response line must be
"HTTP/1.1 200 OK".

https://bugzilla.gnome.org/show_bug.cgi?id=697084
Do not use g_warning() when receiving malformed SSDP messages

Use g_debug() instead of  g_warning() on non-200 responses and
unparsable headers: Scary warnings aren't useful for things we
can't control.

https://bugzilla.gnome.org/show_bug.cgi?id=697084