Fixes issue with collisionCategories and collisionMasks being cleared

* Fixes an issue where the collisionCategories and collisionMasks are cleared and never set again, after saving, if they are not a string

Bug report:
http://forum.spritebuilder.com/t/categories-and-masks-cleared-when-saving/963
https://github.com/spritebuilder/SpriteBuilder/issues/405