Cast JSONLoader Ints to Doubles Where Needed

I don't know anything. I just took the web_gl_morph_targets_horse example and tried to load [this free model](http://www.blendswap.com/blends/view/74387) after using the threej.js exporter on it. I got a casting error, "type 'int' is not a subtype of type 'double' of 'd'," when doing so. The same model worked fine in the three.js editor.

This PR alleviates the symptoms but I don't know if it's a good fix. Please feel free to disregard it for a more appropriate change.