title: vuetify show-select in data-table not selecting items
body: <p>So, I upgraded from vuetify 1.5 to latest (2.1xx) and get stuck on a few places.
  I have a data-table where I want a "select all" checkbox in the header. 
  I added it with the "show-select" property and what I can see is that as the checkbox, when checked, actually puts all the items in the "selected" v-model.</p>
  
  <p>My problem is that I want to have a template for item-props to customize the appearance of the rows and the checkbox that I bind to "props.selected" does not seem to work. If I check any checkbox on any row the item is <em>not</em> added to my "selected" v-model. </p>
  
  <p>It is <em>only</em> if I use no template at all that I get it to work with the auto-generated checkboxes but this does not suffice for my current demands. In vuetify 1.5 I got it to work but I don't understand how to make it work in the new version.</p>
  
  <pre><code>&lt;template&gt;
  &lt;div&gt;
      &lt;v-data-table 
          hide-default-footer 
          v-model="selected" 
          :sort-desc.sync="sortDescending"
          :sort-by.sync="sortBy"
          :headers="headers" 
          :items="cases" 
          item-key="id" 
          show-select 
          :items-per-page="itemsPerPage"
          class="elevation-0"&gt;
  
          &lt;template v-slot:item="props"&gt;
              &lt;tr&gt;
                &lt;td&gt;
                     &lt;v-checkbox v-model="props.selected" color="nordnetBlue" hide-details &gt;&lt;/v-checkbox&gt;
                  &lt;/td&gt;       
                  &lt;td class="navigation-link" @click="goToCase(props.item)"&gt;{{ concatText( props.item.subject, 20) }}&lt;/td&gt;
                  &lt;td&gt;{{ props.item.createdOn }}&lt;/td&gt;
                  &lt;td&gt;{{ props.item.source }}&lt;/td&gt;
                  &lt;td&gt;{{ !props.item.isSameQueue ? props.item.queueName : '' }}&lt;/td&gt;
              &lt;/tr&gt;
          &lt;/template&gt;
      &lt;/v-data-table&gt;
      &lt;pre class="green--text"&gt;{{selected}}&lt;/pre&gt;
  &lt;/div&gt;
  &lt;/template&gt;
  </code></pre>