Add similar type error as the browser for attempting to use form-data…

… without new.

lib/form_data.js

@@ -10,6 +10,10 @@ var async = require('async');
 
 module.exports = FormData;
 function FormData() {
+  if (!(this instanceof FormData)) {
+    throw new TypeError("Failed to construct 'FormData': Please use the 'new' operator, this object constructor cannot be called as a function.")
+  }
+
   this._overheadLength = 0;
   this._valueLength = 0;
   this._lengthRetrievers = [];