Fix InstanceIdentifier toString()

This fixes the issue where InstanceIdentifier.toString() would remain
empty, causing weird debug statements like:

2014-04-14 02:26:50.284 PDT [nettyThreadgroupModule$NioEventLoopGroupCloseable-6-2] TRACE o.o.c.m.s.d.s.impl.MutableDataTree - Resolving modification apply strategy for

Change-Id: Ibde143c614d49f27385da729ac4054627e601b57
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Fix InstanceIdentifier toString()"