title: Groupby in MongoTemplate returning empty fields
body: <p>I have a collection with fields <code>id, a-int, b-int, c-int, total-int</code>. I am trying to get <code>a, b, c, total</code> but I end up getting just the sum of <code>total</code> and rest of the field values are <code>0, 0, 0</code>. How do I fix this? Expected result from the data sample below <code>10, 20, 30, 300</code></p>
  
  <p>Thanks</p>
  
  <p>Data sample</p>
  
  <pre><code>id,   a,  b,  c, total
  xid, 10, 20, 30, 100
  xid, 10, 20, 30, 200
  
  
  GroupBy groupBy = GroupBy.key("{a : 1, b : 1, c : 1}")
    .initialDocument("{ total: 0 }")
    .reduceFunction("function(obj, result) { result.total += obj.total; }");
  
  GroupByResults&lt;Grouped&gt; results = mongoTemplate.group(Criteria.where("id").is(id),
  TABLE, groupBy, Grouped.class);
  </code></pre>