json.lang: minor improvements (MIME + geoJSON & topoJSON)

According to RFC7159 and IANA, JSON official MIME media type is
application/JSON, better specify it.

GeoJSON (.geo.json or .geojson files) and TopoJSON (.topojson files)
are both JSON based formats. This patch also adds file extension
detection for those two.

https://bugzilla.gnome.org/show_bug.cgi?id=740560