title: Setting homepage in package.json doesn't set PUBLIC_URL on build
body: <p>Following the docs <a href="https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md#building-for-relative-paths" rel="nofollow noreferrer">here (Building for Relative Paths)</a>, so setting homepage field in package.json doesn't set PUBLIC_URL during the build.</p>
  
  <p><strong>Expected behavior</strong></p>
  
  <p><code>%PUBLIC_URL%</code> in <code>build/index.html</code> should be replaced with URL set in package.json's homepage field.</p>
  
  <p><strong>package.json:</strong></p>
  
  <pre><code>"homepage": "https://example.com",
  </code></pre>
  
  <p><strong>build/index.html:</strong></p>
  
  <pre><code>&lt;script type="text/javascript" src="https://example.com/static/js/main.ec7f8973.js"&gt;
  </code></pre>
  
  <p><strong>Actual behavior</strong></p>
  
  <pre><code>&lt;script type="text/javascript" src="/static/js/main.ec7f8973.js"&gt;
  </code></pre>
  
  <p><code>%PUBLIC_URL%</code> will be replaced with nothing.</p>