Fix crash when 'mailbox_in' adds an author

When Commit.email_in() detects that the incoming mailbox message adds one or more new authors to a commit, it expands the authors[] array, initializing each new element with a virgin Attribution in which the date property is None. When it later goes to add the new author, it attempts to compare the incoming date against the date in the virgin Attribution and crashes when Date.__ne__ tries accessing None.timestamp.