Fix Unpacking a map with extra fields

An object that is serialized as a map may fail to deserialize if there is an extra field that is not known by the Unpacker.  The map unpacker doesn't attempt to read the value of an unknown key, causing the unpacker to attempt to read the value as the next key, which may result in exceptions.  This fix adds a "default" case to the unpacker, which calls Unpacker.Skip() on the value of any unknown key.