title: Proxy `changeOrigin` setting doesn't seem to work
body: <p>I'm using Vue CLI 3.0.0 (rc.10) and am running two servers (backend server and WDS) side by side.</p>
  
  <p>I followed <a href="https://cli.vuejs.org/config/#devserver-proxy" rel="noreferrer">the devServer.proxy instructions on the Vue CLI documentation</a> to add a proxy option to my <code>vue.config.js</code>. I also followed <a href="https://github.com/chimurai/http-proxy-middleware#options" rel="noreferrer">the instructions for the http-proxy-middleware library</a> to supplement the options:</p>
  
  <pre><code>module.exports = {
    lintOnSave: true,
    outputDir: '../priv/static/',
    devServer: {
      proxy: {
        '/api': {
          target: 'http://localhost:4000',
          changeOrigin: true,
          ws: true,
        },
      },
    },
  }; 
  </code></pre>
  
  <p>My understanding is that the <code>changeOrigin: true</code> option needs to dynamically change the <code>Origin</code> header on the request to "<a href="http://localhost:4000" rel="noreferrer">http://localhost:4000</a>". However, requests from my app are still being sent from <a href="http://localhost:8080" rel="noreferrer">http://localhost:8080</a> and they trigger CORS blockage:</p>
  
  <pre><code>Request URL: http://localhost:4000/api/form
  Request Method: OPTIONS
  Status Code: 404 Not Found
  Remote Address: 127.0.0.1:4000
  Host: localhost:4000
  Origin: http://localhost:8080 &lt;-- PROBLEM
  </code></pre>
  
  <p>What am I doing wrong?</p>