When a JSON array is parsed, a string value containing a comma and no
single or double quote is kept as-is and added to a list.  The comma
being a delimiter for list items, when the list is serialized to JSON
again, that comma is not treated as a part of the original string, but
as a separator:

```
["a", "b", "c,d"]    -> "a,b,c,d"
["a", "b", "c", "d"] <- "a,b,c,d"
```

Add the comma to the list of forbidden chars in a string to force
quoting of the value, preventing this bug:

```
["a", "b", "c,d"] -> "a,b,\"c,d\""
["a", "b", "c,d"] <- "a,b,\"c,d\""
```

Signed-off-by: Romain Tartière <[email protected]>