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

```


Start by tracing the DOMModel and @byContent behavior described in the issue, using the ExampleModel reproduction and its React component as the entry point. Verify when the content is moved or restored for both title values, then add a regression test showing that mounted content remains in place even when the component does not render it conditionally.

Written by the indexing model from the issue text.