Set placeholder image when resouce is nil

Sources/ImageView+Kingfisher.swift

@@ -58,6 +58,7 @@ extension Kingfisher where Base: ImageView {
                          completionHandler: CompletionHandler? = nil) -> RetrieveImageTask
     {
         guard let resource = resource else {
+            base.image = placeholder
             completionHandler?(nil, nil, .none, nil)
             return .empty
         }

Sources/NSButton+Kingfisher.swift

@@ -54,6 +54,7 @@ extension Kingfisher where Base: NSButton {
                          completionHandler: CompletionHandler? = nil) -> RetrieveImageTask
     {
         guard let resource = resource else {
+            base.image = placeholder
             completionHandler?(nil, nil, .none, nil)
             return .empty
         }
@@ -119,6 +120,7 @@ extension Kingfisher where Base: NSButton {
                                   completionHandler: CompletionHandler? = nil) -> RetrieveImageTask
     {
         guard let resource = resource else {
+            base.alternateImage = placeholder
             completionHandler?(nil, nil, .none, nil)
             return .empty
         }

Sources/UIButton+Kingfisher.swift

@@ -56,6 +56,7 @@ extension Kingfisher where Base: UIButton {
                          completionHandler: CompletionHandler? = nil) -> RetrieveImageTask
     {
         guard let resource = resource else {
+            base.setImage(placeholder, for: state)
             completionHandler?(nil, nil, .none, nil)
             return .empty
         }
@@ -126,6 +127,7 @@ extension Kingfisher where Base: UIButton {
                                    completionHandler: CompletionHandler? = nil) -> RetrieveImageTask
     {
         guard let resource = resource else {
+            base.setBackgroundImage(placeholder, for: state)
             completionHandler?(nil, nil, .none, nil)
             return .empty
         }