Bug fix in recent change in iOSExec.nativeCallback (#78)

Now, callbackFromNative will actually get called async
diff --git a/src/www/ios/ios-wkwebview-exec.js b/src/www/ios/ios-wkwebview-exec.js
index f2992c0..331fe93 100644
--- a/src/www/ios/ios-wkwebview-exec.js
+++ b/src/www/ios/ios-wkwebview-exec.js
@@ -124,7 +124,9 @@
 iOSExec.nativeCallback = function (callbackId, status, message, keepCallback, debug) {
     var success = status === 0 || status === 1;
     var args = convertMessageToArgsNativeToJs(message);
-    Promise.resolve(cordova.callbackFromNative(callbackId, success, status, args, keepCallback)); // eslint-disable-line
+    Promise.resolve().then(function () {
+        cordova.callbackFromNative(callbackId, success, status, args, keepCallback); // eslint-disable-line
+    });
 };
 
 // for backwards compatibility