public void And_connection_string_exists_Then_existing_connection_string_should_be_replaced()
{
var settings = new AppSettings("NonExistingFile", FileOption.None);
// Original value
settings.SetConnectionString("cs", "a");
Assert.AreEqual("a", settings.GetConnectionString("cs"));
// Save with different value
settings.SetConnectionString("cs", "b");
Assert.AreEqual("b", settings.GetConnectionString("cs"));
}