Allow session-level connection description in SDP test

[rainey]

Prevents an exception when given an SDP with only session-level connection line. Checks against session-level connection info if media-level connection info is not present. Adds an error for case when niether is present.

Intended as a fix for #876

[jonathan-r-thorpe]

I would suggest getting rid of the connection_global temp variable in the interest of compactness and understandability.

Suggested change

	connection_global = None
	connection_global_line = re.search(r"c=IN IP[4,6] ([^/\r\n]*)(?:/[0-9]+){0,2}", sdp_global)
	if connection_global_line is not None:
	connection_global = connection_global_line.group(1)
	connection_global_line = re.search(r"c=IN IP[4,6] ([^/\r\n]*)(?:/[0-9]+){0,2}", sdp_global)

[jonathan-r-thorpe]

Suggested change

	if connection_line is None:
	if connection_global is None:
	return False, "SDP has no session-level connection description, and no connection line in media description {}"\
	.format(index)
	elif connection_global != transport_params["destination_ip"]:
	return False, "SDP has no connection line for media description {}, and session destination IP {} does not match transport_params: {}" \
	.format(index, connection_global, transport_params["destination_ip"])
	elif connection_line.group(1) != transport_params["destination_ip"]:
	if connection_line is None:
	if connection_global_line is None:
	return False, "SDP has no session-level connection description, and no connection line in media description {}"\
	.format(index)
	elif connection_global_line.group(1) != transport_params["destination_ip"]:
	return False, "SDP has no connection line for media description {}, and session destination IP {} does not match transport_params: {}" \
	.format(index, connection_global_line.group(1), transport_params["destination_ip"])
	elif connection_line.group(1) != transport_params["destination_ip"]: