Kestrel does not allow spaces after the request line

[Tratcher]

A partner team reported compat issues with a client that is sending a space between the HTTP/1.1 request line and the first header. Kestrel (correctly) rejects this request as invalid. IIS allows this.

IIS allows:

GET / HTTP/1.1
 Host: example.com
User-Agent: curl

But not:

GET / HTTP/1.1
User-Agent: curl
 Host: example.com

They're working to get the client fixed, but it will take a while.

Proposal: Patch 6.0 to allow this under an app context switch but do not forward port the patch to main/7.0.