rss: set pubdate with the last modification date

- Sets the publication date to the last modification date if it exists.

Co-Authored-by: iGor milhit <igor@milhit.ch>

layouts/_default/rss.xml

@@ -29,7 +29,11 @@
     <item>
       <title>{{ .Title }}</title>
       <link>{{ .Permalink }}</link>
+      {{ if (and (isset .Params "lastmod") (gt (dateFormat "2006-01-02" (.Lastmod)) (dateFormat "2006-01-02" (.PublishDate)))) }}
+      <pubDate>{{ .Lastmod.Format "Mon, 02 Jan 2006 15:04:05 -0700" | safeHTML }}</pubDate>
+      {{ else }}
       <pubDate>{{ .PublishDate.Format "Mon, 02 Jan 2006 15:04:05 -0700" | safeHTML }}</pubDate>
+      {{ end }}
       {{ with .Site.Params.author.email }}<author>{{.}}{{ with $.Site.Params.author.name }} ({{.}}){{end}}</author>{{end}}
       <guid>{{ .Permalink }}</guid>
       <description>{{ .Content | html }}</description>