Fix sampleOn for multiple triggers of the same value

[neilpa]

siarhei.barysenka [12:24 PM] @neilpa: Hi! It seems RACMarbles for sampleOn contains an issue: http://neilpa.me/rac-marbles/#sampleOn The event ​2​ should be propagated twice on every tick of sample according to the documentation and real behavior of ReactiveCocoa. From the other side, the illustrated behavior is exactly the same as for sample in RxSwift.