Fix unintentional breakage

Yangtools commit 6e7a42bd344f5928343ad67328bc8a8a734bea2e introduced a
new method which broke the controller-side proxy. Fix it up by simply
forwarning it through the delegate.

Change-Id: I4582acb3a6987d9d87d422a86283984a38cd31ba
Signed-off-by: Robert Varga <rovarga@cisco.com>