improve UTF8 support

[robertoostenveld]

With http://github.com/fieldtrip/fieldtrip we are using jsonlab in the data2bids function. @DidiLamers identified a problem when converting a dataset with an author (which goes in the dataset_description.json) that had a non-ascii character in her name (in fact, it was the letter "í", with an acute accent). The BIDS validator subsequently complained that the json is not UTF8; also opening the json file file in the Atom editor resulted in the character being shown correctly.

It might be that this is solved already in version 2.0; we are now shipping FieldTrip with jsonlab version 1.5.

If the limitation is still there in 2.0, I would like to discuss whether this can be solved on the jsonlab-side.

An alternative that I see is that around https://github.com/fieldtrip/fieldtrip/blob/2a67cb59746eb81fe05bc118c96d5f257a63f51c/data2bids.m#L2208
we introduce some extra code to deal with this, like reading the json file and writing it back with UTF8 encoding.

[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.