Bug 25982: OPAC shelves RSS link output is xml

Bug 25982: OPAC shelves RSS link output is xml

Output rss feed as text/xml

Test plan:
1) have books entered
2) log in create a list
3) add books to list
4) display list in OPAC
5) click the RSS link button.
   -- output is displayed as html text
6) apply patch
7) repeat steps 4&5
   -- output is displayed as xml tree

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Amit Gupta <amit.gupta@informaticsglobal.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>