perf: improve allocations in `OwinEnvironment`

[DeagleGross]

One of the internal teams reported testing plain ASP.NET vs ASP.NET with Owin, and they found a big loss in the RPC with the OWIN usage (much bigger difference than expected).

@deanward81 has found the improvement to make in OwinEnvironment, where there is a way to reduce multiple allocations per request.