Fix JDBC response for delete query

[abbashus]

Issue: #131

Description of changes:
Fixes the JDBC output format of delete query

Previously

==== _opendistro/_sql?_format=jdbc ====
{
  "error": {
    "reason": "There was internal problem at backend",
    "details": "class com.amazon.opendistroforelasticsearch.sql.domain.Delete cannot be cast to class com.amazon.opendistroforelasticsearch.sql.domain.Select (com.amazon.opendistroforelasticsearch.sql.domain.Delete and com.amazon.opendistroforelasticsearch.sql.domain.Select are in unnamed module of loader java.net.FactoryURLClassLoader @46963479)",
    "type": "ClassCastException"
  },
  "status": 500
}

Now

{
  "schema": [{
    "name": "deleted_rows",
    "type": "long"
  }],
  "total": 1,
  "datarows": [[1]],
  "size": 1,
  "status": 200
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[dai-chen]

LGTM. Thanks!

[davidcui1225]

LGTM