title: element ui: unable to call resetField to reset single number inputs?
body: <p>I want to reset single number input fields in element ui...</p>
  
  <p>According to the documentation there should be the method resetField:
  <a href="https://element.eleme.io/#/en-US/component/form#form-item-methods" rel="nofollow noreferrer">https://element.eleme.io/#/en-US/component/form#form-item-methods</a></p>
  
  <p>My number inputs are part of a ruleForm following the example code:</p>
  
  <p>I tried:</p>
  
  <pre><code>this.ruleForm.survivalYears.resetField()
  </code></pre>
  
  <p>and</p>
  
  <pre><code>this.$refs['ruleForm'].survivalYears.resetField()
  </code></pre>
  
  <p>What am I doing wrong? My current dirty hack looks like this:</p>
  
  <pre><code>this.ruleForm.survivalYears = -1 // assign some value to rerender
  delete this.ruleForm.survivalYears
  </code></pre>