Fix invalid type of engines during merge result

Engines was stored in set and casted in list, it leads to
issue during merge of result. This patch fixes the engines
container to a list to be able to use json serialization.

Below the error spotted:
```
   File "/usr/local/searx/searx/results.py", line 233, in _merge_result
     duplicated['engines'].add(result['engine'])
 AttributeError: 'list' object has no attribute 'add'
```