title: django-elasticsearch-dsl Mapping for completion suggest field not working
body: <p>This problem has been driving me crazy for days with not solution. </p>
  
  <p>I create a document as follows from my django model.</p>
  
  <p>from django_elasticsearch_dsl import fields</p>
  
  <pre><code>@registry.register_document class QuestionDocument(Document):
      complete = fields.CompletionField(attr='title')
  
      class Index:
          name = 'questions'
  
      class Django:
          model = QuestionModel
          fields = ['text', 'title']
  </code></pre>
  
  <p>Now i want to perform a completion query like this: </p>
  
  <pre><code>matched_questions = list(QuestionDocument.search().suggest("suggestions", word, completion={'field': 'complete'}).execute())
  </code></pre>
  
  <p>But i keep getting the following error: </p>
  
  <pre><code>elasticsearch.exceptions.RequestError: RequestError(400, 'search_phase_execution_exception', 'Field [complete] is not a completion suggest field')
  </code></pre>
  
  <p>I think the problem is that The mapping for this field is not created correctly, but i don't know how to fix it. Can anybody help with this it is literally driving me crazy.</p>
  
  <p><strong>UPDATE:</strong> </p>
  
  <p>I realized that in my mapping, complete is created as a text field, and i don't know why this is happening or how to fix this. This is my mapping:</p>
  
  <pre><code>{
    "questions" : {
      "mappings" : {
        "doc" : {
          "properties" : {
            "complete" : {
              "type" : "text",
              "fields" : {
                "keyword" : {
                  "type" : "keyword",
                  "ignore_above" : 256
                }
              }
            },
            "text" : {
              "type" : "text",
              "fields" : {
                "keyword" : {
                  "type" : "keyword",
                  "ignore_above" : 256
                }
              }
            },
            "title" : {
              "type" : "text",
              "fields" : {
                "keyword" : {
                  "type" : "keyword",
                  "ignore_above" : 256
                }
              }
            }
          }
        }
      }
    }
  }
  </code></pre>