Service URL: keep blank GET arguments

When a service URL contains GET arguments with no associated value, eg
  http://example.com/?foo=a&bar
only the arguments with value are kept, yielding
  http://example.com/?foo=a&ticket=<TICKET>
losing `bar` in the process

cas_server/utils.py

@@ -259,7 +259,7 @@ def update_url(url, params):
             value = value.encode('utf-8')
         params[key] = value
     url_parts = list(urlparse(url))
-    query = dict(parse_qsl(url_parts[4]))
+    query = dict(parse_qsl(url_parts[4], keep_blank_values=True))
     query.update(params)
     # make the params order deterministic
     query = list(query.items())