Fix invalid pubDate (#1425)

根据 https://travis-ci.org/DIYgod/RSSHub/builds/478233072 修正了大部分无效的 pubDate

prprpr

lib/routes/gitea/blog.js

@@ -60,7 +60,7 @@ module.exports = async (ctx) => {
                     title: title,
                     link: link,
                     guid: link,
-                    pubDate: pubDate,
+                    pubDate: pubDate.toUTCString(),
                     description: `<p>${author}</p>${desc}`,
                 };
                 return Promise.resolve(single);