title: Firebase cannot get the last insert value
body: <p>I have a small firebase database with a list. How can I get the last inserted value of the list?
  The elements of the list have as key a date string, example:</p>
  
  <pre><code> 2016_05_29 --&gt; data
   2016_05_30 --&gt; data
   2016_05_31
   2016_06_01
   2016_06_02
   2016_06_03
   2016_06_10
   2016_06_11
   2016_06_12
   2016_06_13
   2016_06_14
   2016_06_15
   2016_06_16
   2016_06_17
   2016_06_18
   2016_06_19 //returned value
   2016_06_20
   2016_06_21
   2016_06_22
   2016_06_23 //expected value
  </code></pre>
  
  <p>I am try to get the last value of this list, so 2016_06_23 but for some reason it returns this 2016_06_19 whatever key/order system I use:</p>
  
  <pre><code>Query dateMaxRef = ref.orderByKey().limitToLast(1); //this returns always 2016_06_19 instead of 2016_06_23 
  dateMaxRef.addListenerForSingleValueEvent(new ValueEventListener() 
  </code></pre>
  
  <p>....</p>
  
  <p>Any idea?</p>