Sending queries through POST, while better for privacy, breaks functionality
with certain extensions (e.g. Firefox containers). Since Firefox does
not send cookies when requesting `/opensearch.xml`, users cannot easily
switch to GET on the client side unless they make a custom search
engine. This commit allows admins to modify the default method on their
side so they can set it to GET if needed.
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.
Mohamad Safadieh