Fixed dumping json - added copying string workaround.

Updated json stat docs. 
I do not even consider that read-only filesystem should be somehow shown in the data.stat file.
Afterall, how can you put that into into stat file if filesystem is in read-only mode?
That's why I asked about *real* problem you are trying to solve, not the idea of 'having this might help'.

I would recommend not to touch this at all. **At most** add single field showing the time of the last successful data.stat file update.