minor fix of BASE engine

searx/engines/base.py

 def response(resp):
     results = []
 
-    search_results = etree.XML(resp.text)
+    search_results = etree.XML(resp.content)
 
     for entry in search_results.xpath('./result/doc'):
         content = "No description available"

tests/unit/engines/test_base.py

         self.assertRaises(AttributeError, base.response, '')
         self.assertRaises(AttributeError, base.response, '[]')
 
-        response = mock.Mock(text='<response></response>')
+        response = mock.Mock(content=b'<response></response>')
         self.assertEqual(base.response(response), [])
 
-        xml_mock = """<?xml version="1.0"?>
+        xml_mock = b"""<?xml version="1.0"?>
 <response>
   <lst name="responseHeader">
     <int name="status">0</int>
   </result>
 </response>"""
 
-        response = mock.Mock(text=xml_mock.encode('utf-8'))
+        response = mock.Mock(content=xml_mock)
         results = base.response(response)
         self.assertEqual(type(results), list)
         self.assertEqual(len(results), 1)