BUG-2304 Fix subtree filter in netconf-impl for identityrefs.

There was a bug in prefixed content comparison.

Change-Id: I5815b9bef404c0d3785b52a9450cb667ac798b9f
Signed-off-by: Maros Marsalek <mmarsale@cisco.com>
Merge "BUG-2304 Fix subtree filter in netconf-impl for identityrefs." into stable/helium