Fix 0 length container handling in StreamingUnpacker. Fix #38

Un-initialized array and map are serialized to 'nil' in pack.
But Unpacker should deserialize 0 length array and map for manual
'beginMap(0)' and 'beginArray(0)' call.
Merge pull request #39 from msgpack/fix-0-length-container

Fix 0 length container handling in StreamingUnpacker. Fix #38