Invoke-WebRequest/Invoke-RestMethod: PR #10034 violates RFC7230 and therefore breaks HTTP Desync Mitigation modes #15186

Background

PR #10034 that fixes issue #9473 and has been introduced in 7.0.0-preview.2.
This sets a String.Empty body for GET requests in Invoke-WebRequest and Invoke-RestMethod which usually do not have a body.

Unfortunately this also leads .NET to add a Content-Length: 0 header because of the String.Empty HTTP request body.

And this breaks RFC7230, see the official RFC7230 Section 3.3.2: https://tools.ietf.org/html/rfc7230#section-3.3.2

   A user agent SHOULD NOT send a
   Content-Length header field when the request message does not contain
   a payload body and the method semantics do not anticipate such a
   body.

Why is this a problem?

• To prevent HTTP Request Smuggling attacks Cloud Service Providers such as AWS and vendors of Application Load Balancers have implemented so-called "HTTP Desync Mitigation modes".
• On AWS the "Strictest Mode" for example blocks all requests that are not RFC7230 conform.
  ** See https://docs.aws.amazon.com/elasticloadbalancing/latest/application/application-load-balancers.html#desync-mitigation-mode for explanation
• Here is a list of classification reasons that are not RFC7230 conform: https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-access-logs.html#classification-reasons
  ** GetHeadZeroContentLength - There is a Content-Length header with a value of 0 for a GET or HEAD request.
  ** This classifies the request as Acceptable which is being blocked in "Strictest Mode"
• You cannot use Invoke-WebRequest or Invoke-RestMethod on AWS hosted websites where this setting has been set
• This will result in a HTTP 400 - Bad Request
• This might also affect other Cloud Service Providers like Azure or Google Cloud Platform, but I cannot confirm that, as I'm only using AWS.

Steps to reproduce

• On an AWS Application Load Balancer set the HTTP Desync Mitigation Mode to "Strictest Mode"
• Then try a simple Invoke-WebRequest on PowerShell 7.0.0-preview.2 or above:

MY C:\> Invoke-WebRequest -Uri "https://mywebsite.example.com/"
Invoke-WebRequest:
400 Bad Request
400 Bad Request

Expected behavior

• Do not set a Content-Length: 0 header as it is not RFC7230 conform if you do not have a HTTP Request body

Actual behavior

• PowerShell 7 always sets the HTTP header Content-Length: 0 for GET/HEAD requests without HTTP Request body and therefore violates RFC7230

Environment data

• This is reproducible on AWS Lambda with PowerShell 7.0.0 on Linux as well as on Windows 10 with PowerShell 7.1.0