fix: improve es5 compatibility (#2, #9)

Author: pi0

lib/index.js

@@ -12,8 +12,7 @@ function defu (_obj, _defaults) {
   }
 
   var obj = Object.assign({}, _defaults)
-
-  Object.keys(_obj).forEach((key) => {
+  Object.keys(_obj).forEach(function (key) {
     if (key === '__proto__' || key === 'constructor') {
       return
     }
@@ -30,7 +29,6 @@ function defu (_obj, _defaults) {
       obj[key] = val
     }
   })
-
   return obj
 }