Join strokes when the first and last points coincide, instead of capping both

[tdewolff]

This will join the last point with the first point of a stroked path. Instead of capping both, which will give undesired results (try ButtCapper for example), the path is now properly joined so that a RoundJoiner will give the expected results.

@nigeltao

[tdewolff]

To clarify, this changes

to

(used together with #49)