DOM content retrieved by @byContent still renders with the react component mounted

### Expected Behaviour
As stated in the documentation:

> The DOM content will be moved when the React component is mounted. And, the content will be put back in its original location if the React component is later unmounted.

### Actual Behaviour
After some tests, if the property holding the content (inside the react component) is not used, even if the react component is still mounted, the DOM content is still put back in its original location.

#### Sample Code that illustrates the problem
###### React component:
```javascript
import React from 'react';
import PropTypes from 'prop-types';

const Example = ({
// products,
title,
content,
}) => (

Example.propTypes = {
// products: PropTypes.arrayOf(PropTypes.object).isRequired,
title: PropTypes.string,
content: PropTypes.oneOfType([
PropTypes.arrayOf(PropTypes.node),
PropTypes.node,
]).isRequired,
};

Example.defaultProps = {
title: 'no',
};

export default Example;
```

###### React container:
```javascript
import {
DOMModel,
byAttrVal,
byContent,
} from '@adobe/react-webcomponent';
import { connect } from 'react-redux';
import Example from '../example/example';
import { action } from 'actions';

const mapStateToProps = state => ({
products: state.products,
});

const ReduxedExample = connect(
mapStateToProps,
{ action },
)(Example);

class ExampleModel extends DOMModel {
@byAttrVal('title') title;

@byContent('.content') content;
}

// Register web component "example-component"
// using above Model
});
```

###### Html example:
```html

```


###### Other html example:
```html

```


Commencez par suivre le comportement de DOMModel et de @byContent décrit dans l’issue, en utilisant la reproduction avec ExampleModel et son composant React comme point d’entrée. Vérifiez quand le contenu est déplacé ou restauré pour les deux valeurs de title, puis ajoutez un test de régression montrant que le contenu monté reste en place même lorsque le composant ne l’affiche pas conditionnellement.

Rédigé par le modèle d'indexation à partir du texte de l'issue.