Changed value for maxDepth to prevent ArgumentOutOfRangeException("maxDepth") to be thrown from ctor SiteMapNodeModel
Using correct variable to add descendants to node.children
Bugfix => descending to a lower level wasn't guarde properly.

Unittests to support correct behaviour when looking for Nodes to include in the model when visibilityAffectsDescendants is set to false and descending to a lower level