Setting `{dynamic_type}` as part of the mapping

[Mpdreamz]

Reported on stackoverflow: https://stackoverflow.com/questions/44933356/create-a-dynamic-type-mapping-in-elasticsearch-with-nest

The user is not able to generate the following using our .Generic construct that can map dynamic mappings like these.

"mapping": {
    "type": "{dynamic_type}",
    "index": "not_analyzed",
    "doc_values": true
}

[lakario]

Thanks for creating a Github issue to track this.