Remove dependency on is-buffer (#1816)

* Remove dependency on is-buffer from package.json

lib/utils.js

@@ -1,7 +1,6 @@
 'use strict';
 
 var bind = require('./helpers/bind');
-var isBuffer = require('is-buffer');
 
 /*global toString:true*/
 
@@ -19,6 +18,17 @@ function isArray(val) {
   return toString.call(val) === '[object Array]';
 }
 
+
+/**
+ * Determine if a value is a Buffer
+ *
+ * @param {Object} val The value to test
+ * @returns {boolean} True if value is a Buffer, otherwise false
+ */
+function isBuffer(val) {
+  return ![undefined, null].includes(val) && val.constructor === Buffer;
+}
+
 /**
  * Determine if a value is an ArrayBuffer
  *