Fix for GeometryCollection not being a valid geometry

[robpc]

GeometryCollection was not properly marshalling when placed in a Feature. Changes added the right annotation.

Example of a JSON message that was not marshaling correctly before the change:

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "geometry": {
        "type": "GeometryCollection",
        "geometries": [
          {
            "type": "Point",
            "coordinates": [100.0, 0.0]
          }
        ]
      }
    }
  ]
}

[grundid]

Thank you