HttpResponseMessage Serialisation still breaks in .NET Core 3.0

[aliostad]

This is related to https://github.com/dotnet/corefx/issues/31918 and the same repro described there breaks with .NET 3.0 but the reason appears to be different and due to not using TryAddWithoutValidation according to this.

Basic issue is that headers such as Expires: -1 are not supported while they are still prevalent.

[aliostad]

[jkonecki]

This issue has been present since .NET Core 2.1!

@aliostad keeps opening new bugs after we are told the issue has been fixed. Could someone PLEASE take a look at it and sort it out once and for all?!

[dougbu]

c55dd95