improve UTF8 support about jsonlab

[fangq]

@robertoostenveld, sorry for leaving this issue open for such a long time. I have not yet been able to figure out an environment to reproduce this issue, but I can vaguely see why this happens.

In JSONLab, strings are saved in MATLAB's "native" encoding - so, it does not guarantee to be utf-8. Strictly speaking this is not JSON compliant, but it is a common practices in many JSON parsers (especially in the non-strict mode).

MATLAB's default text encoding can be queried and set via feature('DefaultCharacterSet'). In my case (Ubuntu Linux), this has been utf-8, but other users may have different settings (or manually set it differently to utf-8). In that case, the string typed in matlab may not be utf-8, and thus could cause the issue.

I suppose this can be resolved by adding

if(varargin{1}.strictunicode)
    val=native2unicode(val);
end

after this line, and add a new option StrictUnicode at the beginning.

However, I could not figure out how to test this. Using a unicode file sample, I was able to print the unicode properly regardless of my DefaultCharacterSet setting. see below screenshot.

if you can help me find a system/matlab configuration that I can reproduce this issue, I would test my above proposed fix.