fix(sitemap): use date only in <lastmod> (#94)

* make the date formate right

experiment: ACCEPT by baidu and google after modified
### Reference:
[baidu](https://ziyuan.baidu.com/college/courseinfo?id=267&page=2#h2_article_title16)
[google](https://support.google.com/webmasters/answer/183668?hl=en&ref_topic=4581190)
[sitemap](https://www.sitemaps.org/protocol.html)
[bing](https://www.bing.com/webmaster/help/how-to-submit-sitemaps-82a15bd4)
[yandex](https://yandex.ru/support/webmaster/controlling-robot/sitemap.html)

* refactor: move date formatting into a dedicated function

Co-authored-by: MDLeom <43627182+curbengh@users.noreply.github.com>

lib/template.js

@@ -18,6 +18,10 @@ module.exports = function(config) {
     return encodeURL(str);
   });
 
+  env.addFilter('formatDate', input => {
+    return input.toISOString().substring(0, 10);
+  });
+
   const sitemapSrc = config.sitemap.template || join(__dirname, '../sitemap.xml');
   sitemapTmpl = nunjucks.compile(readFileSync(sitemapSrc, 'utf8'), env);
 

sitemap.xml

@@ -4,9 +4,9 @@
   <url>
     <loc>{{ post.permalink | uriencode }}</loc>
     {% if post.updated %}
-    <lastmod>{{ post.updated.toISOString() }}</lastmod>
+    <lastmod>{{ post.updated | formatDate }}</lastmod>
     {% elif post.date %}
-    <lastmod>{{ post.date.toISOString() }}</lastmod>
+    <lastmod>{{ post.date | formatDate }}</lastmod>
     {% endif %}
   </url>
   {% endfor %}

test/index.js

@@ -58,7 +58,7 @@ describe('Sitemap generator', () => {
     const { items } = await p(result.data);
     items.forEach((element, index) => {
       element.link.should.eql(posts[index].permalink);
-      element.date.should.eql(posts[index].updated.toISOString());
+      element.date.should.eql(posts[index].updated.toISOString().substring(0, 10));
     });
   });