made a minor change to _zoomOrSpiderfy to Spiderfy without zooming 

If everything in the cluster is at the same lat long. This prevents zooming into max level if there is no real reason to do so, since all points will be at the same location. This minor change increased usability in a personal project.

Thought you guys might appreciate it. This is my first pull request ever, so if I am doing this totally wrong, please let me know.