Fix Json rendering so that special characters don't get HTML-escaped

Previously, if you had certain special characters in JSON Strings (e.g. ' or = or >), they would be HTML-escaped when their Column Type was set to Json.

So, the Json String "It's going to be hot today" would appear as "It\u0027s going to be hot today". [It would of course be rendered correctly if you changed the Column Type to String.]

The fix is simply to call disableHtmlEscaping() on the GsonBuilder before parsing the json strings.