[FIX] Cannot provide default message type in context

mail.message's default_get compares any value of 'default_type' in the context to the entries in the type field's selection in order to throw out invalid types. However, it compares the value (e.g. 'comment') to each tuple, e.g.: ('comment', 'Comment'). So the test always fails and any valid value is always discarded. Present in 7.0 and 8.0.