Bug 11307: Fix potential XSS attack in public catalog RSS feed

Bug 11307: Fix potential XSS attack in public catalog RSS feed

To test:
1/ Craft a url like
/cgi-bin/koha/opac-search.pl?q=a&count=50"'<h1>test</h1>&sort_by=acqdate_dsc&format=rss2
2/ look at the source, notice
<opensearch:itemsPerPage>50"'<h1>test</h1></opensearch:itemsPerPage>
3/ apply the patch, and reload url
4/ source now contains
 <opensearch:itemsPerPage>50&quot;'&lt;h1&gt;test&lt;/h1&gt;</opensearch:itemsPerPage>

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 682e706a4ac10b416b51bdb1ea8894dbe21b345e)
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>