Default model values are passed for attributes that are not present

### Expected Behaviour

If I have defined a model where I specify a boolean attribute. e.g. `@byBooleanAttrVal() checked`. And, I have an instance of my web component where I do not specify that attribute (e.g. . The props passed to my React components `render` method contain the `checked` prop set to the default value from the model definition.

### Actual Behaviour

I would expect any model attributes that are not specified to not be passed as props. Otherwise, I have no way of telling whether the user set that attribute or not.

### Reproduce Scenario (including but not limited to)

I was wrapping up a React implemented checkbox widget.

#### Steps to Reproduce

1. Create a React component that takes a boolean prop
2. Create a model for that component that uses `@byBooleanAttrVal()`
3. Instantiate the web component without using the boolean attribute from your model
4. Set a breakpoint in the `render` method of your React component
5. Look at the props and see that the boolean prop has been passed in

#### Platform and Version

Chrome 72

#### Sample Code that illustrates the problem

#### Logs taken while reproducing problem

Empieza reproduciendo el escenario documentado de custom-element y render de React, centrándote en cómo se recopilan los atributos del modelo y se pasan como props. Se considera completado cuando un atributo booleano omitido está ausente de las props de React, mientras que un atributo especificado explícitamente sigue llegando al render con su valor esperado.

Escrito por el modelo de indexación a partir del texto del issue.