Instant payments going from pending to denied

Hi!

I wanted to handle instant payments that go from pending to denied, and I noticed that they are currently flagged as duplicates. The attached commit makes it so they are not flagged, and I can now handle them through the payment_was_successful signal by checking if ipn_obj.payment_status == ST_PP_DENIED.

I'd be interested to know what you think about this issue and my change.

Cheers.