How to validate on checkbox ?

Hi, I have `checkbox` using `paper-checkbox` and validation form using `ember-cp-validation`, but I get the `form.isInvalid` set to `true` even if I haven't click to checkmark it ?

model

```
import DS from 'ember-data';
import attr from 'ember-data/attr';
import { validator, buildValidations } from 'ember-cp-validations';
import { computed } from '@ember/object';

const { Model } = DS;

const Validations = buildValidations({
premiumType: validator('presence', true),
isAcceptance: validator('presence', true)
});

export default Model.extend(Validations, {
premiumType: attr('string'),
isAcceptance: attr('string')
});
```

template

```
{{#paper-form as |form|}}
{{paper-input label="Premium" value=model.premiumType onChange=(action (mut model.premiumType)) errors=model.validations.attrs.premiumType.messages}}
{{paper-checkbox label="Required" value=model.isAcceptance onChange=(action (mut model.isAcceptance)) errors=model.validations.attrs.isAcceptance.messages}}

{{#form.submit-button disabled=form.isInvalid primary=true raised=true}}
Submit
{{/form.submit-button}}
{{/paper-form}}
```

What I would expect is: the submit button should be Enabled when the validations are valid; but I get the the button already Enabled even if the checkbox has not been checked... Any help?

The issue provides only model and template snippets; start by tracing ember-cp-validations' presence validator and paper-checkbox's value/onChange behavior. Confirm how the unchecked state is represented and how form.isInvalid is derived, then verify that the submit button is disabled until isAcceptance is checked.

由索引模型根据 Issue 内容生成。