title: Firebase query endAt() except/exclude its key
body: <pre><code>a
  b
  c
  d
  e
  </code></pre>
  
  <p><code>FirebaseRef.orderByKey().endAt('c').limitToLast(2)</code> will return </p>
  
  <p><code>b, c</code></p>
  
  <p>I want to retreive except <code>c</code>, so <code>a, b</code> are what I want. </p>
  
  <p>How can I do this?</p>