[2.2.x] Call Flush With fMoreData=false when response isn't allowed to have body

[jkotalik]

For #4398

@muratg @Eilon for patch consideration.

Description:

Today, when and HTTP/2 request is sent to ANCM Inprocess, on a 204 response, IIS will add the header "transfer-encoding"="chunked". This is an invalid response and HTTP.Sys aborts the response with HTTP_1_1_REQUIRED. Web browsers like chrome explicitly show the error HTTP_1_1_REQUIRED when returning a 204. Other browser still have the issue, but they retry a HTTP/1.1 request afterwards.

This means that all HTTP responses which:

• Set a status code of 204
• Use HTTPS
• Support HTTP/2

will explicitly fail in chrome and silently fail/retry in other browsers.

Customer Impact

If customers want to use the 204 response code, they will have to add a middleware to set ContentLength=0 on responses with status code 204.

For example:

app.Use(async (ctx, next) =>
{
  await next();
  if (ctx.Response.StatusCode == 204)
  {
    ctx.Response.ContentLength = 0;
  }
});

Regression?

Regression from all of our server behavior (Kestrel, ANCM + Kestrel).

Risk:

Low. The change only affects 204, 205, and 304 status code responses, which aren't allowed to have a body anyways. This change only sets a bool to false on flush, which is checked when IIS is determining if it should set the encoding to chunked or not.

[natemcmaster]

Approving from an infrastructure-only perspective. The IISIntegration group of projects is already patching in 2.2.1, so no additional infrastructure changes are needed.