title: ElasticSearch 5.1 Fielddata is disabled in text field by default [ERROR: trying to use aggregation on field]
body: <p>Having this field in my mapping</p>
  
  <pre><code>"answer": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
  </code></pre>
  
  <p>i try to execute this aggregation </p>
  
  <pre><code>"aggs": {
  "answer": {
    "terms": {
      "field": "answer"
    }
  },
  </code></pre>
  
  <p>but i get this error</p>
  
  <pre><code>"type": "illegal_argument_exception",
        "reason": "Fielddata is disabled on text fields by default. Set fielddata=true on [answer] in order to load fielddata in memory by uninverting the inverted index. Note that this can however use significant memory."
  </code></pre>
  
  <p>Do i have to change my mapping or am i using the wrong aggregation ? (just updated from 2.x to 5.1)</p>