Constant values in expressions don't abide by the case sensitivity option

[bijington]

It seems that the majority of constant values can be case insensitive apart from null. Really the parser should abide by the ExpressiveOptions.IgnoreCase flag however this could now be a breaking change. For now it is likely that we just make null case insensitive to at least be consistent with the rest.

A test to prove the issue:

[TestMethod]
public void ShouldHandleNullCaseInsensitive()
{
    var arguments = new Dictionary<string, object>
    {
        ["plate.datecontrol"] = null,
    };

    Assert.AreEqual("Date Needed", new Expression("if([plate.datecontrol] = NULL, 'Date Needed', 'Date Entered')", ExpressiveOptions.IgnoreCase).Evaluate(arguments));
}