Proxies broken in .NET Core 3.0

[brettsam]

My analysis from an internal investigation (this is mostly a Teams transcript that I've cleaned up)

We end up with read-only headers when we try to set them in proxies -- this behavior is new in ASP.NET Core 3.0.

This PR changed it: dotnet/aspnetcore#7110. Specifically -- https://github.com/aspnet/AspNetCore/pull/7110/files#diff-6f529bca1b5cb1bf98dbd8b32d27969d.

Notice that right there it now calls response.StartAsync() -- that didn't use to happen; it just wrote the body.

The summary:
Once we call the localFunctionResult, the response is started and headers are marked as read-only. This is new in .NET Core 3: link to location in internal repo.

so when we go to run the proxyResult -- the headers are locked and it throws: link to location in internal repo.

[brettsam]

Proxies was fixed and we've taken the fix with #5050