OpenSearch allows you to (for example) add your site search to the search engines of your browser. Right now, this does not work and has been broken for about a year (See 00d8bac)
This PR fixes the OpenSearchDescription.
How to test
I discovered this issue while working on com_finder for 4.0 and I also fixed the issue there, too. So while you are at it, it would be really nice if you could test #20936 also.