[fix] simple theme: fix autocomplete

AJAX requests send the X-Requested-With HTTP header,
so searx.webapp.autocompleter returns the results with the expected data format.

Related to #2127
Close #2203

searx/static/themes/simple/js/searx_src/searx_search.js

@@ -75,6 +75,10 @@
           Url: "./autocompleter",
           EmptyMessage: searx.no_item_found,
           HttpMethod: searx.method,
+          HttpHeaders: {
+            "Content-type": "application/x-www-form-urlencoded",
+            "X-Requested-With": "XMLHttpRequest"
+          },
           MinChars: 4,
           Delay: 300,
         }, "#" + qinput_id);