Fix a bug:we can use 'Custom Class' in CocosBuilder, and assign custom properties to it, but when you create some nodes with the same custom_class , and they have different custom properties, as they are same class and the share one Loader, so in the loader _customProperties will be wrong.
So, I dropped some lines to clear _customProperties before handle an new node.
Merge pull request #4168 from xhcnb/fix_ccb_nodeloader

[ci skip]Fix a bug:we can use 'Custom Class' in CocosBuilder, and assign custom p...