When the user add searx as a search engine, the browser loads the /opensearch.xml URL without the cookies.
Without the query parameters, the user preferences are ignored (method and autocomplete).
In addition, opensearch.xml is modified to support automatic updates,
see https://developer.mozilla.org/en-US/docs/Web/OpenSearch
Sending query params over GET seems to be the only way to be able to
enable autocomplete in the browser. This commit adds the necessary URL
formatting to opensearch.xml. In order to identify queries coming from
the URL bar (rather than an AJAX request), which requires a different
JSON format and MIME type, the request headers are checked for
"X-Requested-With: XMLHttpRequest" which is added by jQuery request.
