title: React router warning: Received `true` for a non-boolean attribute `exact`
body: <p>I receive console warning during Buttons mapping with prop "exact":</p>
  
  <blockquote>
    <p>Warning: Received <code>true</code> for a non-boolean attribute <code>exact</code>.</p>
    
    <p>If you want to write it to the DOM, pass a string instead:
    exact="true" or exact={value.toString()}.</p>
  </blockquote>
  
  <p>I guess there are no mistakes in my code</p>
  
  <pre class="lang-js prettyprint-override"><code>const TOP_LEVEL_ROUTES = [
    {
      name: 'Home',
      path: HOME_URL,
      component: HomeView,
      exact: true
    },
    {
      name: 'Table',
      path: TABLE_URL,
      component: TableView
    },
    {
      name: 'About',
      path: ABOUT_URL,
      component: AboutView
    }
  ];
  </code></pre>
  
  <pre><code>import React from 'react';
  import { Link } from 'react-router-dom';
  import AppBar from '@material-ui/core/AppBar';
  import Toolbar from '@material-ui/core/Toolbar';
  import Button from '@material-ui/core/Button';
  
  import { LOGO_URL, HOME_URL, TOP_LEVEL_ROUTES } from 'consts';
  import styles from './Header.scss';
  
  const Header = () =&gt; {
    const navigationList = TOP_LEVEL_ROUTES
      .map(({ exact, path, name }) =&gt; (
        &lt;Button
          component={Link}
          to={path}
          key={path}
          exact={exact}
        &gt;
          {name}
        &lt;/Button&gt;
      ));
  
    return (
      &lt;AppBar className={styles.header}&gt;
        &lt;Toolbar className={styles.headerToolbar}&gt;
          &lt;Link to={HOME_URL}&gt;
            &lt;img
              src={LOGO_URL}
              alt='FCIT logo'
            /&gt;
          &lt;/Link&gt;
          &lt;nav className={styles.headerNavbar}&gt;
            {navigationList}
          &lt;/nav&gt;
        &lt;/Toolbar&gt;
      &lt;/AppBar&gt;
    );
  };
  
  export default Header;
  </code></pre>