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

```


Beginne damit, das im Issue beschriebene Verhalten von DOMModel und @byContent nachzuverfolgen, wobei die Reproduktion mit ExampleModel und deren React-Komponente als Einstiegspunkt dient. Überprüfe, wann der Inhalt für beide title-Werte verschoben oder wiederhergestellt wird, und füge anschließend einen Regressionstest hinzu, der zeigt, dass gemounteter Inhalt an seiner Stelle bleibt, selbst wenn die Komponente ihn nicht bedingt rendert.

Vom Indexierungsmodell aus dem Issue-Text verfasst.