fix: .rss2() not support Extension

[Yrobot]

Describe the bug

feed.addExtension({
      name: "ext",
      objects: {
         value: 2
      },
});

the ext data will be there in the .json1() result, but not in the result of .rss2().

Expected behavior

<ext> should be in the result of .rss2()

Actual behavior
<ext> is not in the result of .rss2()

#198