title: @model decorator from vue-property-decorator generates vue-warn: Avoid mutating
body: <p>I use typescript with <a href="https://github.com/kaorun343/vue-property-decorator" rel="noreferrer">this</a> lib. And I have following code in file <code>HomePage.vue</code> :</p>
  
  <pre><code>&lt;template&gt;
      &lt;div&gt;Write something x: &lt;input v-model="someName" type="text"&gt;&lt;/div&gt;
  &lt;/template&gt;
  
  &lt;script lang="ts"&gt;
  import { Component, Model, Vue } from "vue-property-decorator";
  
  @Component
  export default class HomePage extends Vue {
      @Model() someName: string;
  }
  &lt;/script&gt;
  </code></pre>
  
  <p>After compilation, run in browser and type something on input box i get in chrome console following vue warrning:</p>
  
  <p><em>[Vue warn]: Avoid mutating a prop directly since the value will be overwritten whenever the parent component re-renders. Instead, use a data or computed property based on the prop's value. Prop being mutated: "someName"</em></p>
  
  <p>Any idea how to fix this problem?</p>