notificationCenter public var added

Reachability/Reachability.swift

@@ -71,6 +71,8 @@ public class Reachability {
     public var whenReachable: NetworkReachable?
     public var whenUnreachable: NetworkUnreachable?
     public var reachableOnWWAN: Bool
+    
+    public var notificationCenter: NotificationCenter = NotificationCenter.default
 
     public var currentReachabilityString: String {
         return "\(currentReachabilityStatus)"
@@ -236,7 +238,7 @@ fileprivate extension Reachability {
         let block = isReachable ? whenReachable : whenUnreachable
         block?(self)
         
-        NotificationCenter.default.post(name: ReachabilityChangedNotification, object:self)
+        self.notificationCenter.post(name: ReachabilityChangedNotification, object:self)
         
         previousFlags = flags
     }