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."
Merge branch 'master' of github.com:nunit/nunit-framework