title: Element Ui component is not rerendering on vue component prop change
body: <p>I have a parent component and multiple child components, which use the same prop. This prop is an array of keys for a dropdown menu in element.js.
  When the children render the first time, they contain no data. However, once the keys from arrive using vuefire the children get the dropdown menu items. However, the element dropdown menu is not rerendered as it should have been.
  However using the vue dev tools, I can see that the dropdown menu entries have been passed down as a key. When vue does a hot reload, because of a file change, the keys will load.
  Once the entries are loaded, I can select the entry and everything works as expected.</p>
  
  <p>I also had the same results using the vuetify dropdown and the HTML dropdown. Both have the same issue. </p>
  
  <p>parent</p>
  
  <pre><code>&lt;template&gt;
    &lt;div class="setup"&gt;
      &lt;h1&gt;Setup&lt;/h1&gt;
      &lt;div class="selectIngredients" v-for="number in 6"&gt;
        &lt;setupSelection :bottle="number" :ingredients="options" /&gt;
      &lt;/div&gt;
    &lt;/div&gt;
  &lt;/template&gt;
  
  &lt;script&gt;
  import {db} from "@/firebaseConfig"
  import setupSelection from '@/components/setupSelection';
  export default {
    components: {
      setupSelection,
    },
    firestore: {
      options: db.collection('ingredients'),
    },
  };
  &lt;/script&gt;
  </code></pre>
  
  <p>child</p>
  
  <pre><code>&lt;template&gt;
    &lt;div class="ingredientSelector"&gt;
      &lt;h3&gt;Select for Pump &lt;span&gt;{{bottle}}&lt;/span&gt;&lt;/h3&gt;
      &lt;el-select v-model="selected" clearable placeholder="Select" &gt;
        &lt;el-option
          v-for="ingredient in ingredients"
          v-bind:key="ingredient.text"
          v-bind:label="ingredient.text"
          v-bind:value="ingredient"&gt;
        &lt;/el-option&gt;
      &lt;/el-select&gt;
       &lt;!-- &lt;v-select
          v-model="selected"
          :items="ingredients"
          label="Select a favorite activity or create a new one"
        &gt;&lt;/v-select&gt; --&gt;
      &lt;!-- &lt;select v-model="selected" v-for="ingredient in ingredients"&gt;
        &lt;option :value="ingredient.value"&gt;{{ingredient.text}}&lt;/option&gt;
      &lt;/select&gt; --&gt;
    &lt;/div&gt;
  &lt;/template&gt;
  
  &lt;script&gt;
  import {db} from "@/firebaseConfig";
  export default {
    props: {
      ingredients: { required: true },
      bottle: { type: Number, required: true },
    },
    data() {
      return {
        selected: ''
      }
    },
    },
  };
  &lt;/script&gt;
  </code></pre>
  
  <p>I expected the dropdown menu to update once the client received them.</p>
  
  <p>Thank you!</p>