Dict compliant behaviour.

see #585

modoboa/extensions/webmail/views.py

@@ -335,7 +335,7 @@ def listmailbox(request, defmailbox="INBOX", update_session=True):
     :return: a dictionnary
     """
     navparams = WebmailNavigationParameters(request, defmailbox)
-    previous_page_id = int(navparams["page"])
+    previous_page_id = int(navparams["page"]) if "page" in navparams else None
     if update_session:
         navparams.store()
     mbox = navparams.get('mbox')

modoboa/lib/webutils.py

@@ -178,11 +178,13 @@ class NavigationParameters(object):
     def __getitem__(self, key):
         """Retrieve an item."""
         if self.sessionkey not in self.request.session:
-            return None
+            raise KeyError
         return self.request.session[self.sessionkey][key]
 
     def __contains__(self, key):
         """Check if key is present."""
+        if self.sessionkey not in self.request.session:
+            return False
         return key in self.request.session[self.sessionkey]
 
     def __setitem__(self, key, value):