[fix] improve OpenSearch description

Some HTTP-Clients do have issues with the ``opensearch.xml`` from SearXNG
(related [1][2]) while other OpenSearch descriptions[3] (e.g. from qwant) work
flawles.

Inspired by the OpenSearch description from qwant and with informations from the
specification[4] the ``opensearch.xml`` has been *improved*.

- convert `<Url>` methods from lower case to upper case (`POST`|`GET`)
- add `<moz:SearchForm>` and `xmlns:moz="http://www.mozilla.org/2006/browser/search/"`
- add `<Query role="example" searchTerms="SearXNG" />`  [4]

  OpenSearch description documents should include at least one Query element of
  `role="example"` that is expected to return search results. Search clients may
  use this example query to validate that the search engine is working properly.

- modified `<LongName>` to SearXNG
- modified `<Description>` the word 'hackable' scares uninitiated users and was removed
- add the `type="image/png"` to `<Image>`

Test can be done by::

    make run

Visit http://127.0.0.1:8888/ and add the search engine to your WEB-Browser /
test with different WEB-Browser from desktop and Smartphones (are there any iOS
user here, please test on Safari and Chrome).

[1] https://app.element.io/#/room/#searxng:matrix.org/$xN_abdKhNqUlgXRBrb_9F3pqOxnSzGQ1TG0s0G9hQVw
[2] https://github.com/searxng/searxng/issues/431
[3] https://developer.mozilla.org/en-US/docs/Web/OpenSearch
[4] https://github.com/dewitt/opensearch/blob/master/opensearch-1-1-draft-6.md#the-query-element

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/webapp.py

@@ -1282,19 +1282,17 @@ Disallow: /*?*q=*
 
 @app.route('/opensearch.xml', methods=['GET'])
 def opensearch():
-    method = 'post'
-
-    if request.preferences.get_value('method') == 'GET':
-        method = 'get'
+    method = request.preferences.get_value('method')
+    autocomplete = request.preferences.get_value('autocomplete')
 
     # chrome/chromium only supports HTTP GET....
     if request.headers.get('User-Agent', '').lower().find('webkit') >= 0:
-        method = 'get'
+        method = 'GET'
 
-    autocomplete = request.preferences.get_value('autocomplete')
+    if method not in ('POST', 'GET'):
+        method = 'POST'
 
     ret = render('opensearch.xml', opensearch_method=method, autocomplete=autocomplete)
-
     resp = Response(response=ret, status=200, mimetype="application/opensearchdescription+xml")
     return resp