feat: add pubDate to RSS feed items

lib/volumen/server.rb

@@ -299,16 +299,29 @@ module Volumen
 
     def feed_item(post)
       link = "#{base_url}/#{post.slug}"
+      pub = if post.metadata["date"]
+              "\n      <pubDate>#{escape(rfc822_date(post))}</pubDate>"
+            else
+              ""
+            end
       <<~XML.chomp
         <item>
           <title>#{escape(post.title)}</title>
           <link>#{escape(link)}</link>
           <guid>#{escape(link)}</guid>
-          <description>#{escape(post.excerpt)}</description>
+          <description>#{escape(post.excerpt)}</description>#{pub}
         </item>
       XML
     end
 
+    def rfc822_date(post)
+      value = post.metadata["date"]
+      case value
+      when Date then value.to_time.rfc822
+      when Time then value.rfc822
+      end
+    end
+
     def sitemap_xml
       urls = published_posts.map do |post|
         "  <url><loc>#{escape("#{base_url}/#{post.slug}")}</loc></url>"