title: How to set indeterminate state in vuetify checkbox?
body: <p>I have a checkbox in Vuetify and at first it has an indeterminate state. After I change it to true or false, I have a button to reset it to default and show the indeterminate state again but it doesnt work. What's wrong?
  the defalut value for indeterminate is true and value is null.</p>
  
  <pre><code>&lt;v-checkbox
    :indeterminate="indeterminate"
    v-model="value"
  &gt;&lt;/v-checkbox&gt;
  </code></pre>
  
  <p>and the button does this:</p>
  
  <pre><code>&lt;v-btn @click="setToDefault()"&gt; set to default &lt;/v-btn&gt;
  </code></pre>
  
  <p>method:</p>
  
  <pre><code>    setToDefault(){
        this.value = null
        this.indeterminate = true
       }
  </code></pre>