Ways for polygons rings should be closed

[gidema]

A Way primitive that represents a inner or outer ring of a polygon should be closed. This means the start and end of the ring should share the same node.
Currently, the plug-in doesn't support this. A solution could be to add a method createClosedWay to the DataSetBuilder class and use this method instead of createWay when building polygons.

[don-vip]

DataSetBuilder is gone with #16. Can you please check if it works now?

[floscher]

With #26, ways are now closed if the start node and end node have exactly the same coordinates.