Bug 16111: Fix content type for RSS feed

Bug 16111: Fix content type for RSS feed

Test plan:
GET http://catalogue/cgi-bin/koha/opac-search.pl?idx=kw&q=perl&count=50&sort_by=acqdate_dsc&format=rss2

=> Without this patch it returns content type text/html
=> With this patch it returns text/xml

GET http://catalogue/cgi-bin/koha/opac-search.pl?idx=kw&q=perl&count=50&sort_by=acqdate_dsc&format=rss
=> Without this patch it does not work
=> With this patch it returns text/xml

Bug 16111: Fix atom support
Bug 16111: Remove 'rss2' which should be 'rss' or 'atom'
Bug 16111: Replace <link> with <link href...

Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit b946e996ff208e71bbfd5cfb7714f9a656065093)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>