BUG-1414: fix InstanceIdentifier.child(Class, Object)

The result of the method is always a KeyedInstanceIdentifier, so let's
not force users to use casts.

Change-Id: Icabb8999445ff64936e7a32a69eacfc2c9647c64
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1414: fix InstanceIdentifier.child(Class, Object)"