results: make engines of results set

Previously, if a same result was returned from the
same engine, the name of the engine would be added
more than once. To eliminate engine name duplication
engine list was changed to set.

searx/results.py

             result['parsed_url'] = result['parsed_url']._replace(scheme="http")
             result['url'] = result['parsed_url'].geturl()
 
-        result['engines'] = [result['engine']]
+        result['engines'] = set([result['engine']])
 
         # strip multiple spaces and cariage returns from content
         if result.get('content'):
             duplicated['positions'].append(position)
 
             # add engine to list of result-engines
-            duplicated['engines'].append(result['engine'])
+            duplicated['engines'].add(result['engine'])
 
             # using https if possible
             if duplicated['parsed_url'].scheme != 'https' and result['parsed_url'].scheme == 'https':