title: django_elasticsearch_dsl 'search_phase_execution_exception', 'Field [name] is not a completion suggest field'
body: <p>I'm trying to perform Completion with django-elasticsearch-dsl. So i created the following document:</p>
  
  <pre><code>@registry.register_document
  class QuestionDocument(Document):
      name = fields.TextField(
          attr='title',
          fields={
              'suggest': fields.Completion(),
          }
      )
  
      class Index:
          name = 'questions'
  
      class Django:
          model = QuestionModel
          fields = ['text', 'title']
  </code></pre>
  
  <p>And to perform completion with elasticsearch:</p>
  
  <pre><code>matched_questions = list(QuestionDocument.search().suggest("suggestions", word, completion={'field': 'name.suggest'}).execute())
  </code></pre>
  
  <p>But the following exception occurs saying that no mapping exists for name.suggest:</p>
  
  <pre><code>elasticsearch.exceptions.RequestError: RequestError(400, 'search_phase_execution_exception', 'no mapping found for field [name.suggest]')
  </code></pre>
  
  <p>What am i doing wrong here? Is my approach correct or should i be doing this in a complete different way?</p>