Bug 8678: XSLT stylesheets output XML, but HTML is needed

Bug 8678: XSLT stylesheets output XML, but HTML is needed

Because we are embedding the results of XSLT stylesheets in HTML, we
need to output HTML instead of XML. Outputting XML results in
non-standard-compliant (or at least non-Firefox-compliant) markup.

To test:
View a variety of XSLT-rendered pages on the OPAC and intranet, and
confirm that all look as expected.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>