Update the test cases and readme about AnimatedImage

Author: dreampiggy

README.md

@@ -180,7 +180,11 @@ var body: some View {
         AnimatedImage(name: "animation1", isAnimating: $isAnimating)) // Animation control binding
         .maxBufferSize(.max)
         .onViewUpdate { view, context in // Advanced native view coordinate
+            // AppKit Tool Tip
             view.toolTip = "Mouseover Tip"
+            // UIKit advanced content mode
+            view.contentMode = .topLeft
+            // Coordinator, used for Cocoa Binding or Delegate method
             let coordinator = context.coordinator
         }
     }
@@ -191,6 +195,8 @@ Note: `AnimatedImage` supports both image url or image data for animated image f
 
 Note: `AnimatedImage` some methods like `.transition`, `.indicator` and `.aspectRatio` have the same naming as `SwiftUI.View` protocol methods. But the args receive the different type. This is because `AnimatedImage` supports to be used with UIKit/AppKit component and animation. If you find ambiguity, use full type declaration instead of the dot expression syntax.
 
+Note: some of methods on `AnimatedImage` will return `some View`, a new Modified Content. You'll lose the type related modifier method. For this case, you can either reorder the method call, or use Native View `.onViewUpdate` for rescue.
+
 ```swift
 var body: some View {
     AnimatedImage(name: "animation2") // Just for showcase, don't mix them at the same time

Tests/AnimatedImageTests.swift

@@ -163,6 +163,9 @@ class AnimatedImageTests: XCTestCase {
             XCTAssertEqual(context.coordinator.userInfo?["foo"] as? String, "bar")
         }
         .placeholder(PlatformImage())
+        .placeholder {
+            Circle()
+        }
         .indicator(SDWebImageActivityIndicator.medium)
         // Image
         .resizable()

Tests/WebImageTests.swift

@@ -82,6 +82,7 @@ class WebImageTests: XCTestCase {
         .onProgress { _, _ in
 
         }
+        .placeholder(.init(platformImage: PlatformImage()))
         .placeholder {
             Circle()
         }