need to add test for dynamic updates

TODO.markdown

@@ -5,6 +5,7 @@
 * allow multiple edns0 options to exist in the record when converting
   from/to wireformat
 * NSEC3 support propper in the zone structure(s)
+* Test all rdata packing with zero rdata -- allowed for dynamic updates
 
 ## Nice to have
 

msg.go

@@ -464,6 +464,7 @@ func packStructValue(val reflect.Value, msg []byte, off int, compression map[str
 					return lenmsg, &Error{Err: "overflow packing a"}
 				}
 			case `dns:"aaaa"`:
+				// fv.Len TODO(mg) dynamisc updates?
 				if fv.Len() > net.IPv6len || off+fv.Len() > lenmsg {
 					return lenmsg, &Error{Err: "overflow packing aaaa"}
 				}