Bug 11994: OpenSearch plugins needs to be delivered with Content-Type application...

Bug 11994: OpenSearch plugins needs to be delivered with Content-Type application/opensearchdescription+xml

https://developer.mozilla.org/en-US/docs/Web/OpenSearch
Your server should serve OpenSearch plugins using
Content-Type: application/opensearchdescription+xml.

To test:
- Verify the Content-Type the file is delivered with
  is correct, for example using wget
  http://127.0.0.1:8080/cgi-bin/koha/opac-search.pl?format=opensearchdescription

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>