Change _propagateEvent to be careful that it doesn't overwrite e.layer

if it is already set. Allows markers to be in multiple FeatureGroups and have the events come through correctly. See the test for details.

Fixes leaflet/Leaflet.markercluster#128


The old behaviour was broken in this case (e.layer would become a FeatureGroup). I can't think of any case the new behaviour would be undesirable, but maybe you can mourner?
Thanks :)