ox-rss: Encode characters to their XML entities

* contrib/lisp/ox-rss.el (org-rss-build-channel-info, org-rss-headline):
  Encode disallowed characters in `title' to their XML entities.

The `title' field is user specified and may contain characters such as
"&", "<" or ">" that are disallowed in XML.  These characters should be
encoded into their corresponding XML entities.

contrib/lisp/ox-rss.el

@@ -246,12 +246,13 @@ communication channel."
 			    (format-time-string
 			     "%a, %d %b %Y %H:%M:%S %z"
 			     (org-time-string-to-time pubdate0)))))
-	     (title (or (org-element-property :RSS_TITLE headline)
-			(replace-regexp-in-string
-			 org-bracket-link-regexp
-			 (lambda (m) (or (match-string 3 m)
-					 (match-string 1 m)))
-			 (org-element-property :raw-value headline))))
+	     (title (org-rss-plain-text
+		     (or (org-element-property :RSS_TITLE headline)
+			 (replace-regexp-in-string
+			  org-bracket-link-regexp
+			  (lambda (m) (or (match-string 3 m)
+					  (match-string 1 m)))
+			  (org-element-property :raw-value headline))) info))
 	     (publink
 	      (or (and hl-perm (concat (or hl-home hl-pdir) hl-perm))
 		  (concat
@@ -316,7 +317,7 @@ as a communication channel."
 (defun org-rss-build-channel-info (info)
   "Build the RSS channel information."
   (let* ((system-time-locale "C")
-	 (title (plist-get info :title))
+	 (title (org-export-data (plist-get info :title) info))
 	 (email (org-export-data (plist-get info :email) info))
 	 (author (and (plist-get info :with-author)
 		      (let ((auth (plist-get info :author)))