Fix copyright field rendering HTML in RSS feed (#1479)

The copyright is rendered in footer as `{{ site.Copyright | markdownify }}`
which assumes it to be Markdown text, but RSS feed template doesn't ready
for Markdown now.

layouts/_default/rss.xml

@@ -59,7 +59,7 @@
     <language>{{ site.Language.LanguageCode }}</language>{{ with $authorEmail }}
     <managingEditor>{{.}}{{ with $authorName }} ({{ . }}){{ end }}</managingEditor>{{ end }}{{ with $authorEmail }}
     <webMaster>{{ . }}{{ with $authorName }} ({{ . }}){{ end }}</webMaster>{{ end }}{{ with site.Copyright }}
-    <copyright>{{ . }}</copyright>{{ end }}{{ if not .Date.IsZero }}
+    <copyright>{{ . | markdownify | plainify | strings.TrimPrefix "© " }}</copyright>{{ end }}{{ if not .Date.IsZero }}
     <lastBuildDate>{{ (index $pages.ByLastmod.Reverse 0).Lastmod.Format "Mon, 02 Jan 2006 15:04:05 -0700" | safeHTML }}</lastBuildDate>{{ end }}
     {{- with .OutputFormats.Get "RSS" }}
     {{ printf "<atom:link href=%q rel=\"self\" type=%q />" .Permalink .MediaType | safeHTML }}