title: Firestore batch set is only inserting the last item
body: <p>I'm trying to batch create a bunch of documents from the array items, where each item is a map of key-value pairs. I've got multiple values in my array, but for some reason the only item that ever gets inserted into the db is the last item in the array.</p>
  
  <p>What am I doing wrong here? Or can I only use setData once in the history of the commit?</p>
  
  <pre><code>let wordsRef = db.collection("items").document()
  
  for item in items {
      batch.setData(item, forDocument: wordsRef)
  }
  
  batch.commit() { err in
      // error reporting
  }
  </code></pre>
  
  <p>Thanks heaps!</p>