util: dont repeat isBuffer

PR-URL: https://github.com/nodejs/io.js/pull/1988
Reviewed-By: Roman Reiss <me@silverwind.io>

lib/util.js

@@ -640,10 +640,7 @@ function isPrimitive(arg) {
 }
 exports.isPrimitive = isPrimitive;
 
-function isBuffer(arg) {
-  return arg instanceof Buffer;
-}
-exports.isBuffer = isBuffer;
+exports.isBuffer = Buffer.isBuffer;
 
 function objectToString(o) {
   return Object.prototype.toString.call(o);