title: v-data-table ignores template when updating from Vuetify 1.5 => 2
body: <p>I had a working v-data-table, and was able to customize the displayed rows with a template. That code no longer works once updating to vuetify 2.0 (codepen link below). I'm assuming something has changed with vuetify, but I'm not sure what.  </p>
  
  <p><a href="https://codepen.io/hurleyrc89/pen/KKPWWpQ" rel="nofollow noreferrer">Example</a></p>
  
  <pre><code>&lt;v-data-table
      :headers="headers"
      :items="desserts"
      class="elevation-1"&gt;
    &lt;template v-slot:items="props"&gt;
      &lt;td&gt;{{ props.item.name }}&lt;/td&gt;
      &lt;td class="text-xs-right"&gt;{{ props.item.calories }}&lt;/td&gt;
      &lt;td class="text-xs-right"&gt;{{ props.item.fat }}&lt;/td&gt;
    &lt;/template&gt;
  &lt;/v-data-table&gt;
  </code></pre>
  
  <p>In the example provided, I would only expect the first two columns to show (as it did in v1.5), however the default v-data-table view with all columns is still showing. Please help!</p>