Fixed failing test on machine with german culture

Errors and Failures -
1) Failed :
NUnit.Framework.Syntax.ThrowsTests.LambdaThrowsExceptionWithMessage
Expected: instance of <System.ArgumentNullException> and property
Message String matching "null"
But was:  "Der Wert darf nicht NULL sein."
Revert "fixed failing test on non-US machines"

This reverts commit 5f6b4a8f8e5f9cbe25b47b90da7e10328d7342c9.