title: Why I do not receive after_key from my composite aggregation query?
body: <p>Elastic search Version: 6.2.4.
  My query</p>
  
  <pre><code>"aggs": {
          "composite_agg": {
            "aggs": {
              "ActDate": {
                "max": {
                  "field": "details.details.ActDate",
                  "format": "yyyy'-'MM'-'dd'T'HH':'mm':'ss.SSSSSSSZZ"
                }
              }
            },
            "composite": {
              "size": 2,
              "sources": [
                {
                  "OrgId": {
                    "terms": {
                      "field": "details.details.OrgId"
                    }
                  }
                }
              ]
            }
          }
        }
  </code></pre>
  
  <p>The result of the query</p>
  
  <pre><code> "aggregations": {
          "composite_agg": {
            "buckets": [
              {
                "key": {
                  "OrgId": 16777217
                },
                "doc_count": 40,
                "ActDate": {
                  "value": 1554408754291,
                  "value_as_string": "2019-04-04T20:12:34.2910000+00:00"
                }
              },
              {
                "key": {
                  "OrgId": 16777218
                },
                "doc_count": 1,
                "ActDate": {
                  "value": 1554402199081,
                  "value_as_string": "2019-04-04T18:23:19.0810000+00:00"
                }
              }
            ]
          }
        }
  </code></pre>
  
  <p>Why I do not receive after_key to get more items.
  There are more items than I got in my query and this I see if I increase the size parameter.</p>