[fix] use english as default language in bing

If no language is specified, bing returns results with multiple languages
for one query which isn't really useful. Setting english as default
insted if nothing.

searx/engines/bing.py

     offset = (params['pageno'] - 1) * 10 + 1
 
     if params['language'] != 'all':
-        query = u'language:{} {}'.format(params['language'].split('-')[0].upper(),
-                                         query.decode('utf-8')).encode('utf-8')
+        lang = params['language'].split('-')[0].upper()
+    else:
+        lang = 'EN'
+
+    query = u'language:{} {}'.format(lang, query.decode('utf-8')).encode('utf-8')
 
     search_path = search_string.format(
         query=urlencode({'q': query}),

tests/unit/engines/test_bing.py

 
         dicto['language'] = 'all'
         params = bing.request(query, dicto)
-        self.assertTrue('language' not in params['url'])
+        self.assertTrue('language' in params['url'])
 
     def test_response(self):
         self.assertRaises(AttributeError, bing.response, None)