support files= in StubTreq

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Currently if you specify a files= argument to StubTreq, it complains that this is not supported.

The reason it's not supported is that HTTPClient.request, when given a files= argument, will construct a treq.multipart.MultiPartProducer without giving it a cooperator= argument; the default value for cooperator= is twisted.internet.task, which populates MultiPartProducer._cooperate with twisted.internet.task.cooperate, which implicitly references the global reactor.

It should be possible to push down a Cooperator object through HTTPClient so that the instantiated MultiPartProducer can use the passed in one. (Given that Cooperator is itself a stateful scheduler, it's not quite enough just to pass an IReactorTime.)

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Related: #161