Possible streaming body divergence on Windows

[aturon]

In https://github.com/fastly/Viceroy/pull/188/, we found that Windows behaves differently than Unix-based platforms with respect to streaming bodies. In particular, the test tries to carefully construct a situation where we expect backpressure on a streaming body, and on Windows that backpressure seems to be cleared without either server or client side taking explicit action.

It's not clear whether this behavior difference stems from the client or the server side; in both cases we are using Hyper.