RemoveLayers() when spiderified bug fix

Here is a fix for a bug related to an old bug (see #49).
You can see an example here: http://jsfiddle.net/Grsmto/wSJSF/1/
Just spiderfy the marker, then click on the third button (it will trigger a removeLayers()). You can see that the unspiderfy doesn't happens correctly.

I also added a test for this case.