title: How to setSummary of a preference-headers
body: <p>How can I set a summary of a preference-header on runtime? Even if I put a key in it, <code>findPreference(key)</code> will produce a NullPointerException. </p>
  
  <pre><code>&lt;preference-headers xmlns:android="http://schemas.android.com/apk/res/android"&gt;
      &lt;header android:fragment="PrefActivity$Fragment1"
              android:title="Fragment1"
              android:summary="Summary2"/&gt;
      &lt;header android:fragment="PrefActivity$Fragment2"
              android:title="Fragment2"
              android:summary="Summary2"/&gt;
  &lt;/preference-headers&gt;
  </code></pre>
  
  <p>Thanks for your help!</p>