Allow getChildNodes() to become a Collection

Preparatory patch for yangtools change -- getChildNodes() will start
returning a Collection, not a Set.

Change-Id: Ie7255757e3f48465e3acd9e65764f1b79a3a9511
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Allow getChildNodes() to become a Collection"