title: Flutter Firestore startAt() endtAt() not working when orderby string values
body: <p>I Am trying to use startAt([1]) and endAt([3]) when order by title but it doesn't work.</p>
  
  <pre><code>QuerySnapshot snapshot = await Firestore.instance.collection('items')
    .orderBy("title", descending: false).startAt([1]).endAt([2]).limit(2).getDocuments();
  </code></pre>
  
  <p>When I try by order by rank startAt([1]) and endAt([3]) it works.</p>
  
  <pre><code>QuerySnapshot snapshot5 = await Firestore.instance.collection('items')
    .orderBy("rank", descending: false).startAt([1]).endAt([2]).limit(2).getDocuments();
  </code></pre>
  
  <p>Works when i don't use endAt()</p>
  
  <p>Why doesn't it work when i order by title? </p>
  
  <pre><code>QuerySnapshot snapshot = await Firestore.instance.collection('items')
    .orderBy("title", descending: false).startAt([1]).limit(2).getDocuments();
  </code></pre>
  
  <p>Anyone have any idea?</p>
  
  <p>What i have tried:
  1. ensure packages are up to date
  2. created indexes on firestore</p>
  
  <p>[THIS DOES NOT WORK]
  QuerySnapshot snapshot = await Firestore.instance.collection('items').orderBy("title", descending: false).startAt([1]).endAt([2]).limit(2).getDocuments();</p>
  
  <pre><code>[THIS WORKS]
  QuerySnapshot snapshot5 = await Firestore.instance.collection('items')
    .orderBy("rank", descending: false).startAt([1]).endAt([2]).limit(2).getDocuments();
  
  [THIS WORKS]
  QuerySnapshot snapshot = await Firestore.instance.collection('items')
    .orderBy("title", descending: false).startAt([1]).limit(2).getDocuments();
  </code></pre>
  
  <p>What i expected was to be able to use endAt() when filtering but its throwing error</p>