WebSession: Python-2.4 combatibility issue fix

* In Python-2.4 tuples don't have the index attribute. Converting a tuple to a
  list will provide that attribute.

Signed-off-by: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>