Add better suppport for nested collections in persistent structs

[samnung]

Hey, I've found that nested collections are not supported the same way I expected so I've made some changes to make it little bit better.

Main problem was that when you save persistent struct twice it saves inserts objects in nested collection twice. So it ends up by having collection with twice more values than you expected :D

[terhechte]

Thanks! Great change. Really awesome. I'll release an update soon, so it is really cool that I can include this change. Cheers.