Update utils.py

Fixing DJANGO_MAILBOX_default_charset error, it does not detect the settings in django correctly because one part is in lower case.
Changed to DJANGO_MAILBOX_DEFAULT_CHARSET.

django_mailbox/utils.py

@@ -64,7 +64,7 @@ def get_settings():
         ),
         'default_charset': getattr(
             settings,
-            'DJANGO_MAILBOX_default_charset',
+            'DJANGO_MAILBOX_DEFAULT_CHARSET',
             'iso8859-1',
         )
     }