fix jexl transformation expression when is a JSON based value

[AlvaroVega]

bug in previos PR #861 by @chicco785
to avoid json expression will be transformed to [object Object]

{
            "name" : "mylocation",
            "type" : "geo:json",
            "expression" : "{type: \"Point\", coordinates: [lat,lon]}"
        }

No CNR update is needed since is reopen a previous PR

[mrutid]

LGTM. Improve Json management is debt.

[chicco785]

i think it may be good to add this in the test and examples

[fgalan]

i think it may be good to add this in the test and examples

Sound goods. Maybe you could help providing such contribution? We can focus our resources in fix some flaws of your original implementation, but for things like those your contribution would be very welcomed!