Use `if case` for element and error properties

RxSwift/Event.swift

@@ -87,12 +87,10 @@ extension Event {
     */
     public var element: Element? {
         get {
-            switch self {
-            case .Next(let value):
+            if case .Next(let value) = self {
                 return value
-            case .Error: fallthrough
-            case .Completed: return nil
             }
+            return nil
         }
     }
     
@@ -101,14 +99,10 @@ extension Event {
     */
     public var error: ErrorType? {
         get {
-            switch self {
-            case .Next:
-                return nil
-            case .Error(let error):
+            if case .Error(let error) = self {
                 return error
-            case .Completed:
-                return nil
             }
+            return nil
         }
     }
 }