Makes `observer` in `Sink` private again.

RxSwift/Observables/Implementations/DistinctUntilChanged.swift

@@ -20,8 +20,6 @@ class DistinctUntilChangedSink<O: ObserverType, Key>: Sink<O>, ObserverType {
     }
     
     func on(event: Event<E>) {
-        let observer = super.observer
-        
         switch event {
         case .Next(let value):
             do {

RxSwift/Observables/Implementations/FlatMap.swift

@@ -77,8 +77,6 @@ class FlatMapSink<SourceType, S: ObservableConvertibleType, O: ObserverType wher
     }
     
     func on(event: Event<SourceType>) {
-        let observer = super.observer
-        
         switch event {
         case .Next(let element):
             do {

RxSwift/Observables/Implementations/Map.swift

@@ -25,8 +25,6 @@ class MapSink<SourceType, O : ObserverType> : Sink<O>, ObserverType {
     }
 
     func on(event: Event<SourceType>) {
-        let observer = super.observer
-        
         switch event {
         case .Next(let element):
             do {

RxSwift/Observables/Implementations/Sink.swift

@@ -9,20 +9,20 @@
 import Foundation
 
 class Sink<O : ObserverType> : SingleAssignmentDisposable {
-    let observer: O
+    private let _observer: O
 
     func forwardOn(event: Event<O.E>) {
         if disposed {
             return
         }
-        observer.on(event)
+        _observer.on(event)
     }
 
     init(observer: O) {
 #if TRACE_RESOURCES
         OSAtomicIncrement32(&resourceCount)
 #endif
-        self.observer = observer
+        _observer = observer
     }
 
     deinit {