GetDescendant for a simple range fails with a HierarchyIdException

[AndyBates6668]

The following unit test fails with the current code:

    [TestMethod]
    public void InsertTopLevelNodeBetweenNodes()
    {
        var id = SqlHierarchyId.GetRoot();
        var idChild = id.GetDescendant(SqlHierarchyId.Parse("/0/"), SqlHierarchyId.Parse("/1/"));
        Assert.AreEqual("/0.1/", idChild.ToString());
    }

[AndyBates6668]

I'll see what I can do to resolve the issue.

[ppasieka]

@AndyBates6668 I have a fix for this issue. Tomorrow I will try to create a PR