title: keep new line in <ExpansionPanelDetails> of material UI
body: <p>I got a expanded panel looks as following:</p>
  
  <pre><code>  &lt;ExpansionPanel&gt;
      &lt;ExpansionPanelSummary&gt;this is a title&lt;/ExpansionPanelSummary&gt;
      &lt;ExpansionPanelDetails&gt;
        &lt;Typography&gt;line 1&lt;/Typography&gt;
        &lt;Typography&gt;line 2&lt;/Typography&gt;
        &lt;Typography&gt;line 3&lt;/Typography&gt;
      &lt;/ExpansionPanelDetails&gt;
    &lt;/ExpansionPanel&gt;
  </code></pre>
  
  <p>I need the contents in <code>ExpansionPanelDetails</code> keep multiple lines:</p>
  
  <pre><code>line 1
  line 2
  line 3
  </code></pre>
  
  <p>But it actually is:</p>
  
  <pre><code>line 1line 2line 3
  </code></pre>
  
  <p>I tried wrap <code>&lt;Typography&gt;</code> with<code>&lt;div&gt;</code> and add <code>&lt;br /&gt;</code>. All couldn't work.</p>
  
  <p>Any ideas? </p>
  
  <p>Thanks!</p>
  
  <p><code>"@material-ui/core": "^3.7.0"</code></p>