Make all flatten strategies available on `PropertyType`.
Make all flatten strategies available on `PropertyType`.

victorlin/ReactiveCocoa,calebd/ReactiveCocoa,andersio/ReactiveCocoa,walkingsmarts/ReactiveCocoa,ohwutup/ReactiveCocoa,sandyway/ReactiveCocoa,andersio/ReactiveCocoa,andersio/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,ohwutup/ReactiveCocoa,calebd/ReactiveCocoa,calebd/ReactiveCocoa,victorlin/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,sandyway/ReactiveCocoa,ohwutup/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,victorlin/ReactiveCocoa,ikesyo/ReactiveCocoa,walkingsmarts/ReactiveCocoa,sandyway/ReactiveCocoa,walkingsmarts/ReactiveCocoa,ikesyo/ReactiveCocoa