check for null parameter as base class does, but throw ArgumentException

The XmlAttributeCollection.SetNamedItem(XmlNode) override does parameter validation but does not check for null.  Parent class does a null check to avoid a null ref exception.  Added a null check to the override but throw an ArgumentException as is done for its other parameter checking.  Also added this condition check to the existing NamedItemDoesNotExist test.
Adding the `node == null` check in this if makes the exception message odd, `SR.Xdom_AttrCol_Object` currently says: `An 'Attributes' collection can only contain 'Attribute' objects.`

Wouldn't it make more sense to put the check in a separate if and throw an ArgumentNullException?