_paused_at exception in stopped download

[warner]

There's a bug in web downloads: when someone halts the download (by hitting Stop on their browser and thus closing the HTTP connection), the client dumps an exception to the twisted log. The problem is that the _paused_at attribute is being used without a guard that makes sure it was first initialized.

I don't think this is likely to cause significant problems, as it occurs after the client has abandoned their download anyways. Worst case it might cause the Download object to stick around forever, but I suspect that it gets gc'ed pretty quickly. So it's probably just noise in the logs.

Both resumeProducing and stopProducing need to update the time-spent-paused timer in clauses that are guarded by if self._paused

[warner]

This was fixed in fd465b4aaf179677, before the 1.2.0 release, but a related problem (self._paused being used during registerProducer before it was initialized) was not fixed until cd4d3cdeeacca0aa, after the 1.2.0 release. So I'll mark this one as fixed, but for the 1.2.1 milestone.