Issue 1161 - creates various ways to trigger the exception

[cosmo0]

This PR is demonstrating the issue #1161 by adding a Minimal Reproductible Example.

This creates three ways to trigger the issue, unfortunately with 3 different results, and I'm not sure whether they are the result of unit tests misconfiguration or other underlying problems:

• The new unit test in MessageHeadersTests fails the comparison because the Header is not filled
• The new unit test in IntegrationTests throws a weird exception wherein it mentions different property names

The third way to trigger the exception is as follows:

• open the solution with Visual Studio
• ensure you have the proper exception settings: menu Debug > Windows > Exception Settings, then check the root of "Common Language Runtime Exceptions"
• right click on the project "SoapCore-Issue-1161" and click "Set as startup project"
• run the project (F5)
• in the project, open the file "SoapCore-Issue-1161.http"
• in the second request, click "send request"

This should put you in the place that throws the exception