Merge pull request #1017 from hoanglm4/master

fix. Crash happens when HeadlessInAppWebView's dispose function is called in iOS

ios/Classes/HeadlessInAppWebView/HeadlessInAppWebView.swift

@@ -92,4 +92,9 @@ public class HeadlessInAppWebView : FlutterMethodCallDelegate {
         HeadlessInAppWebViewManager.webViews.removeValue(forKey: id)
         flutterWebView = nil
     }
+    
+    deinit {
+        print("HeadlessInAppWebView - dealloc")
+        dispose()
+    }
 }