title: vue-router: this.$router.push not working when updating query
body: <p>I want to update query on url. But, $router.push does not updating url. <code>disp</code> is different value for each call. Why?</p>
  
  <pre><code>handle: function(disp) {
      let route = Object.assign({}, this.$route);
      route.query.disp = disp;
      this.$router.push(route);
  },
  </code></pre>
  
  <p>versions.</p>
  
  <pre><code>"vue": "^2.5.17",
  "vue-router": "^3.0.1",
  </code></pre>
  
  <p>route (console.log)</p>
  
  <p><a href="https://i.stack.imgur.com/ouh4V.png" rel="noreferrer"><img src="https://i.stack.imgur.com/ouh4V.png" alt="enter image description here"></a></p>