[fix] hash error if url is including non ascii characters

searx/webapp.py

     if not settings['server'].get('image_proxy') and not request.cookies.get('image_proxy'):
         return url
 
-    h = hashlib.sha256(url + settings['server']['secret_key']).hexdigest()
+    h = hashlib.sha256(url.encode('utf-8') + settings['server']['secret_key'].encode('utf-8')).hexdigest()
 
     return '{0}?{1}'.format(url_for('image_proxy'),
                             urlencode(dict(url=url, h=h)))
     if not url:
         return '', 400
 
-    h = hashlib.sha256(url + settings['server']['secret_key']).hexdigest()
+    h = hashlib.sha256(url.encode('utf-8') + settings['server']['secret_key'].encode('utf-8')).hexdigest()
 
     if h != request.args.get('h'):
         return '', 400