IIS in-proc changes some responses from content-length to chunked

Converting Content-Length responses to Chunked is problematic as clients may be expecting that content-length value.

Frequently reproduces with:
```
ctx.Request.Headers.ContentLength = ctx.Request.Path.Value.Length;
await ctx.Response.WriteAsync(ctx.Request.Path.Value);
```

```
HTTP/1.1 200 OK
Transfer-Encoding: chunked
Server: Microsoft-IIS/10.0
X-Powered-By: ASP.NET
Date: Mon, 03 Jun 2019 16:13:19 GMT

2
/a

```

Reproduces less frequently with:
```
ctx.Request.Headers.ContentLength = ctx.Request.Path.Value.Length;
return ctx.Response.WriteAsync(ctx.Request.Path.Value);
```

@jkotalik

It's also known to change some auto-chunked responses to content-length, but that's less problematic as Content-Length is universally supported.