Return an error if the attribute requires authorization

If an attribute requires authorization, Insuficient Authorization
will be returned by the attribute server until the Agent supports
a method to authorize attribute access.
diff --git a/src/attrib-server.c b/src/attrib-server.c
index 9543da6..dc05d7e 100644
--- a/src/attrib-server.c
+++ b/src/attrib-server.c
@@ -173,6 +173,8 @@
 		channel->encrypted = g_attrib_is_encrypted(channel->attrib);
 	if (reqs == ATT_AUTHENTICATION && !channel->encrypted)
 		return ATT_ECODE_INSUFF_AUTHEN;
+	else if (reqs == ATT_AUTHORIZATION)
+		return ATT_ECODE_INSUFF_AUTHO;
 
 	switch (opcode) {
 	case ATT_OP_READ_BY_GROUP_REQ: