change _addForBounds first argument to hash object

After looking at it more I think this is the better fix for my IE 8 issue.

What happened was IE doesn't have support for Array.prototype.indexOf and somewhere in my code it added a shim for it.  

When addForBounds loops through the layerArray, "indexOf" was returned as one of the keys, then crashes out.

The better fix is to add as a hash object instead of an array on line 22 of ```boundsawarelayergroup.js```.

In Leaflet source code for [LayerGroup](https://github.com/Leaflet/Leaflet/blob/master/src/layer/LayerGroup.js#L9), you can see that 
```
this._layers = {}; 
```