use node Buffer to build MockBuffer, not safe-buffer

Author: Andrey Sidorov

lib/packets/packet.js

@@ -1,5 +1,6 @@
 var ErrorCodeToName = require('../constants/errors.js');
 
+var NativeBuffer = require('buffer').Buffer;
 var Buffer = require('safe-buffer').Buffer;
 var Long = require('long');
 
@@ -805,7 +806,7 @@ Packet.prototype.type = function () {
 Packet.MockBuffer = function () {
   var noop = function () {};
   var res = Buffer.alloc(0);
-  for (var op in Buffer.prototype) {
+  for (var op in NativeBuffer.prototype) {
     if (typeof res[op] == 'function') {
       res[op] = noop;
     }