title: Issue with create index with mapping
body: <p>I'm using elastic search version 7.1.0 and NEST 6.7.0 with Elasticsearch.Net 6.7.0.
  While I'm trying to create an index I've got an error.</p>
  
  <p>This is my code, I was using the same code on NEST and Elasticsearch.Net version 6.4.0 and it was working fine</p>
  
  <pre><code>var result =   Client.CreateIndex(DefaultIndex, c =&gt; c
                  .Settings(s =&gt; s
                      .NumberOfShards(numberOfShards)
                      .NumberOfReplicas(numberOfReplicas)
                      .BlocksReadOnly(false)
                  )
                  .Mappings(m =&gt; m
                      .Map&lt;SearchDocument&gt;(mm =&gt; mm
                          .AutoMap(3)
                      )
                  )
              );
  </code></pre>
  
  <p>This is the error that I got </p>
  
  <blockquote>
    <p>Invalid NEST response built from a unsuccessful low level call on PUT:
    /local_brainbank_index</p>
    
    <h1>Audit trail of this API call:</h1>
    
    <ul>
    <li>[1] BadResponse: Node: <a href="http://localhost:9200/" rel="nofollow noreferrer">http://localhost:9200/</a> Took: 00:00:00.2880040
    
    <h1>OriginalException: Elasticsearch.Net.ElasticsearchClientException: The remote server returned an error: (400) Bad Request.. Call: Status</h1></li>
    </ul>
    
    <p>code 400 from: PUT /local_brainbank_index. ServerError: Type:
    mapper_parsing_exception Reason: "Failed to parse mapping [_doc]: Root
    mapping definition has unsupported parameters:  [searchdocument :
    {properties={sourceId={type=integer}, sourceType={type=text,
    fields={keyword={ignore_above=256, type=keyword}}},
    authorName={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, subject={type=text,
    fields={keyword={ignore_above=256, type=keyword}}},
    imageURL={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, id={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, sourceName={type=text,
    fields={keyword={ignore_above=256, type=keyword}}}, body={type=text,
    fields={keyword={ignore_above=256, type=keyword}}}, uRL={type=text,
    fields={keyword={ignore_above=256, type=keyword}}}}}]" CausedBy:
    "Type: mapper_parsing_exception Reason: "Root mapping definition has
    unsupported parameters:  [searchdocument :
    {properties={sourceId={type=integer}, sourceType={type=text,
    fields={keyword={ignore_above=256, type=keyword}}},
    authorName={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, subject={type=text,
    fields={keyword={ignore_above=256, type=keyword}}},
    imageURL={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, id={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, sourceName={type=text,
    fields={keyword={ignore_above=256, type=keyword}}}, body={type=text,
    fields={keyword={ignore_above=256, type=keyword}}}, uRL={type=text,
    fields={keyword={ignore_above=256, type=keyword}}}}}]"" --->
    System.Net.WebException: The remote server returned an error: (400)
    Bad Request.    at System.Net.HttpWebRequest.GetResponse()    at
    Elasticsearch.Net.HttpWebRequestConnection.Request[TResponse](RequestData
    requestData)    --- End of inner exception stack trace ---</p>
  </blockquote>
  
  <h1>Request:</h1>
  
  <pre><code>{"mappings":{"searchdocument":{"properties":{"id":{"type":"text","fields":{"keyword":{"type":"keyword","ignore_above":256}}},"sourceType":{"type":"text","fields":{"keyword":{"type":"keyword","ignore_above":256}}},"sourceId":{"type":"integer"},"sourceName":{"type":"text","fields":{"keyword":{"type":"keyword","ignore_above":256}}},"authorName":{"type":"text","fields":{"keyword":{"type":"keyword","ignore_above":256}}},"subject":{"type":"text","fields":{"keyword":{"type":"keyword","ignore_above":256}}},"body":{"type":"text","fields":{"keyword":{"type":"keyword","ignore_above":256}}},"uRL":{"type":"text","fields":{"keyword":{"type":"keyword","ignore_above":256}}},"imageURL":{"type":"text","fields":{"keyword":{"type":"keyword","ignore_above":256}}}}}},"settings":{"index.number_of_replicas":0,"index.blocks.read_only":false,"index.number_of_shards":1}}
  </code></pre>
  
  <h1>Response:</h1>
  
  <blockquote>
    <p>{"error":{"root_cause":[{"type":"mapper_parsing_exception","reason":"Root
    mapping definition has unsupported parameters:  [searchdocument :
    {properties={sourceId={type=integer}, sourceType={type=text,
    fields={keyword={ignore_above=256, type=keyword}}},
    authorName={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, subject={type=text,
    fields={keyword={ignore_above=256, type=keyword}}},
    imageURL={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, id={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, sourceName={type=text,
    fields={keyword={ignore_above=256, type=keyword}}}, body={type=text,
    fields={keyword={ignore_above=256, type=keyword}}}, uRL={type=text,
    fields={keyword={ignore_above=256,
    type=keyword}}}}}]"}],"type":"mapper_parsing_exception","reason":"Failed
    to parse mapping [_doc]: Root mapping definition has unsupported
    parameters:  [searchdocument : {properties={sourceId={type=integer},
    sourceType={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, authorName={type=text,
    fields={keyword={ignore_above=256, type=keyword}}},
    subject={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, imageURL={type=text,
    fields={keyword={ignore_above=256, type=keyword}}}, id={type=text,
    fields={keyword={ignore_above=256, type=keyword}}},
    sourceName={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, body={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, uRL={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}}}]","caused_by":{"type":"mapper_parsing_exception","reason":"Root
    mapping definition has unsupported parameters:  [searchdocument :
    {properties={sourceId={type=integer}, sourceType={type=text,
    fields={keyword={ignore_above=256, type=keyword}}},
    authorName={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, subject={type=text,
    fields={keyword={ignore_above=256, type=keyword}}},
    imageURL={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, id={type=text, fields={keyword={ignore_above=256,
    type=keyword}}}, sourceName={type=text,
    fields={keyword={ignore_above=256, type=keyword}}}, body={type=text,
    fields={keyword={ignore_above=256, type=keyword}}}, uRL={type=text,
    fields={keyword={ignore_above=256, type=keyword}}}}}]"}},"status":400}</p>
  </blockquote>