unly monkey-patch MockBuffer property if it's a function

Author: sidorares

lib/packets/packet.js

@@ -684,7 +684,9 @@ Packet.MockBuffer = function () {
   var noop = function () {};
   var res = Buffer(0);
   for (var op in Buffer.prototype) {
-    res[op] = noop;
+    if (typeof res[op] == 'function') {
+      res[op] = noop;
+    }
   }
   return res;
 };