title: v-model implementation via render function is not reactive
body: <p>I am trying to create dynamic input component that will be interchangeable between input and textarea tag. I am trying to implement this by using render function. (<a href="https://v2.vuejs.org/v2/guide/render-function.html#v-model" rel="nofollow noreferrer">https://v2.vuejs.org/v2/guide/render-function.html#v-model</a>).</p>
  <p>The problem I have is that v-model works only one way, if I change data property directly it updates textarea value, but if I change or input new data into textarea it does not update the data property. Does anyone know how to make it work both ways?
  Here is my code link for code pen is bellow it illustrates the problem:</p>
  <pre><code>const tag = Vue.component('dynamic-tag', {
      name: 'dynamic-tag',
      render(createElement) {
          return createElement(
              this.tag,
              {
                  domProps: {
                      value: this.value
                  },
                  on: {
                      input: event =&gt; {
                          this.value = event.target.value
                      }
                  }
              },
              this.$slots.default
          )
      },
      props: {
          tag: {
              type: String,
              required: true
          }
      }
  })
  
  const app = new Vue({
    el: '#app',
    data: {
      message: ''
    },
    components: {tag}
  })
  </code></pre>
  <p><a href="http://codepen.io/asolopovas/pen/OpWVxa?editors=1011" rel="nofollow noreferrer">http://codepen.io/asolopovas/pen/OpWVxa?editors=1011</a></p>