Merge pull request #1084 from moon2l/master

fix bug: TypeError: set(['bing', 'google']) is not JSON serializable,…

searx/webapp.py

                                     'corrections': list(result_container.corrections),
                                     'infoboxes': result_container.infoboxes,
                                     'suggestions': list(result_container.suggestions),
-                                    'unresponsive_engines': list(result_container.unresponsive_engines)}),
+                                    'unresponsive_engines': list(result_container.unresponsive_engines)},
+                                   default=lambda item: list(item) if isinstance(item, set) else item),
                         mimetype='application/json')
     elif output_format == 'csv':
         csv = UnicodeWriter(StringIO())