Handle multigeometry features in geojson

The attached patch contains two little improvements to the geojson module:

• handling feature with multiple geometries
• handling of objects that are not strings nor numbers, dates or booleans

For the first case, the patch makes it so that the other geometries are encoded as properties with the same encoding style as the main geometry.
For the second case, it makes it possible to encode valid json out of a random object by using converters. The old code would have treated the object as a literal and not enclose it in double quotes, resulting in invalid json (the only things not quoted are numbers and booleans)