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In ContactView.form_valid(), the before_send signal is sent while specifying self.__class__ as the sender. In case of subclasses, __class__ is the subclass instead of ContactView.
The signal handler is only connected to the ContactView base class, so subclasses fail to send the signal.
Maybe ContactView should be specified as the sender when sending the signal instead?
A work-around is to register the signal for the subclass as well.
Test case for issue #18.
Check sender class in signal handler (closes #18).