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'
```

searx/results.py

@@ -203,14 +203,14 @@ class ResultContainer(object):
             result['parsed_url'] = result['parsed_url']._replace(scheme="http")
             result['url'] = result['parsed_url'].geturl()
 
-        result['engines'] = set([result['engine']])
+        result['engines'] = [result['engine']]
 
         # strip multiple spaces and cariage returns from content
         if result.get('content'):
             result['content'] = WHITESPACE_REGEX.sub(' ', result['content'])
 
         # check for duplicates
-        duplicated = False
+        duplicated = None
         for merged_result in self._merged_results:
             if compare_urls(result['parsed_url'], merged_result['parsed_url'])\
                and result.get('template') == merged_result.get('template'):
@@ -233,7 +233,8 @@ class ResultContainer(object):
             duplicated['positions'].append(position)
 
             # add engine to list of result-engines
-            duplicated['engines'].add(result['engine'])
+            if result['engine'] not in duplicated['engines']:
+                duplicated['engines'].append(result['engine'])
 
             # using https if possible
             if duplicated['parsed_url'].scheme != 'https' and result['parsed_url'].scheme == 'https':

searx/search_database.py

@@ -118,9 +118,6 @@ def get_search_data(q, r):
         results_number = 0
     results = r.get_ordered_results()
     for result in results:
-        result['engines'] = list(result['engines'])
-        if not type(result['engines']) is list:
-            print(result['engines'])
         if 'publishedDate' in result:
             try:
                 result['pubdate'] = result['publishedDate'].strftime('%Y-%m-%d %H:%M:%S')