When document type is `delete`, there is no need to send `fields` attribute.

In fact, doing so will result in a warning like:

     "message" => "\"delete\" has unknown attribute(s): ['fields'] (near operation with index 1; document_id developmentdrewslzimaclocal67)"
Merge pull request #11 from drewda/document-to-hash

When document type is `delete`, there is no need to send `fields` attribute