Fix error in null checking

lib/request-base.js

@@ -189,20 +189,23 @@ exports.unset = function(field){
  * @api public
  */
 exports.field = function(name, val) {
-  // check
-  if (null === name ||  undefined === val) {
+
+  // name should be either a string or an object.
+  if (null === name ||  undefined === name) {
     throw new Error('.field(name, val) name can not be empty');
   }
-  if (null === val || undefined === val) {
-    throw new Error('.field(name, val) val can not be empty');
-  }
 
   if (isObject(name)) {
     for (var key in name) {
       this.field(key, name[key]);
     }
     return this;
   }
+
+  // val should be defined now
+  if (null === val || undefined === val) {
+    throw new Error('.field(name, val) val can not be empty');
+  }
   this._getFormData().append(name, val);
   return this;
 };