fix: move util.isArray to Array.isArray

lib/form_data.js

@@ -61,7 +61,7 @@ FormData.prototype.append = function(field, value, options) {
   }
 
   // https://github.com/felixge/node-form-data/issues/38
-  if (util.isArray(value)) {
+  if (Array.isArray(value)) {
     // Please convert your array into string
     // the way web server expects it
     this._error(new Error('Arrays are not supported.'));