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.markercluster#128 ( https://github.com/Leaflet/Leaflet.markercluster/issues/128 )
Merge pull request #1359 from danzel/fix-events

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