Subscribe leaks out actual data reference

[ronag]

I noticed while doing the exact opposite, i.e. removing necessary deepCopy, that subscribe actually leaks out _$data instead of calling this.get().

https://github.com/deepstreamIO/deepstream.io-client-js/blob/master/src/record/record.js#L191

                args.callback( this._$data );

should be:

                args.callback( this.get() );

[yasserf]

good catch! wana raise a pr?