Fixed bug in mailbox_name argument and added unit tests

This is an optional positional argument, and thus needs to have nargs='?' set

django_mailbox/management/commands/processincomingmessage.py

@@ -21,6 +21,7 @@ class Command(BaseCommand):
     def add_arguments(self, parser):
         parser.add_argument(
             'mailbox_name',
+            nargs='?',
             help="The name of the mailbox that will receive the message"
         )