GeoJSON: implement Date/Time/DateTime field type detection

git-svn-id: https://svn.osgeo.org/gdal/trunk@28009 f0d54148-0727-0410-94bb-9a71ac55c965
Fix warning in non-debug build

git-svn-id: https://svn.osgeo.org/gdal/trunk@28010 f0d54148-0727-0410-94bb-9a71ac55c965