Allow setting ShowFullTextinRSS in individual pages

Currently only the site-wide ShowFullTextinRSS setting is effective. Use
`.Param` so that one can specify ShowFullTextinRSS for individual posts.

layouts/_default/rss.xml

@@ -73,7 +73,7 @@
       {{- with $authorEmail }}<author>{{ . }}{{ with $authorName }} ({{ . }}){{ end }}</author>{{ end }}
       <guid>{{ .Permalink }}</guid>
       <description>{{ with .Description | html }}{{ . }}{{ else }}{{ .Summary | html }}{{ end -}}</description>
-      {{- if and site.Params.ShowFullTextinRSS .Content }}
+      {{- if and (.Param "ShowFullTextinRSS") .Content }}
       <content:encoded>{{ (printf "<![CDATA[%s]]>" .Content) | safeHTML }}</content:encoded>
       {{- end }}
     </item>