Attribute src is ignored in tag content in config.xml (IOS)

I have the problem only in IOS.

The steps for replicate the problem in a new project are:

```
cordova create testContent
cd testContent/
cordova platform add ios
cordova plugin add cordova-plugin-ionic-webview@latest
```

change the name of file index.html to index-example.html
change the config.xml preference to
``

Build and execute the app, you will get a blank page
`cordova build ios && xed platforms/ios`

If I remove the plugin everything works as expected
`cordova plugin rm cordova-plugin-ionic-webview@latest`