Set session state dirty if updated

When updating session with data from the client, the states must be mark as
dirty to be saved in the database at the end of the process.

issue2421

trytond/wizard/wizard.py

@@ -328,7 +328,10 @@ class Wizard(WarningErrorMixin, URLMixin):
         if isinstance(session, (int, long)):
             session = Session(self, session)
         for key, value in data.iteritems():
+            prev_data = session.data[key].copy()
             session.data[key].update(value)
+            if prev_data != session.data[key]:
+                getattr(session, key).dirty = True
 
         state = self.states[state_name]
         result = {}