title: React-Router not unmounting on location parameter change
body: <p>This problem is driving me insane, I got a simple little app created by <code>create-react-app</code> and I am using <code>React-Router</code>.</p>
  <h2>JSX:</h2>
  <pre><code>&lt;Router&gt;
      &lt;Route path=&quot;/pages/:pageId&quot; component={Page} /&gt;
  &lt;/Router&gt;
  </code></pre>
  <p>In the Page component I have an fetch on <code>componentDidMount</code>, its working as expected first time the route is loaded.</p>
  <p>For example I visit: <code>/pages/13</code> and then it fetches that pages data, but when I am on: <code>/pages/13</code> and then click on a Link component (imported from <code>React-Router</code>) redirecting me to: <code>/pages/15</code> - the <code>pageId</code> prop is then updated but it never re-renders.</p>
  <p>The <code>Page</code> component never run <code>componentWillUnmount</code> before, or <code>componentDidMount</code> after.</p>
  <h3>Expected behavior:</h3>
  <ol>
  <li>Visit: <code>/pages/13</code> → <code>render()</code> → <code>componentDidMount()</code> → <code>render()</code></li>
  <li>Click on Link component <code>/pages/15</code> → <code>componentWillUnmount()</code> → <code>render()</code>
  → <code>componentDidMount()</code> → <code>render()</code></li>
  </ol>
  <hr />
  <p>I am using the following versions:</p>
  <ul>
  <li>react: 16.3.2</li>
  <li>react-dom: 16.3.2</li>
  <li>react-router-dom: 4.2.2</li>
  </ul>
  <hr />
  <p>What am I doing wrong? I am still pretty new to React so be nice :)</p>