Trailing commas break folding

[guhetier]

When there is a trailing comma in a json object, foldings stop at the line with the trailing comma and restart at level one after it.

For instance,

{
  "baz": {
    "foo": "bar",
  }
}

give

+-- 3 lines: {-------------------
  }
}

instead of :

+-- 5 lines: {-------------------

Trailing commas are not valid in the JSON syntax but are accepted by several parser. They are therefore used in a lot of files where folding is currently broken.