Fix nested mapping properties

Properties of nested types should be within a properties object and not directly in the parent.

Current JSON output (incorrect):
```json
{
  "type": {
    "nestedProp": {
      "type": "nested",
      "nested1": { "type": "string" },
      "nested2": { "type": "long" }
    }
  }
}
```

Correct JSON:
```json
{
  "type": {
    "nestedProp": {
      "type": "nested",
      "properties": {
        "nested1": { "type": "string" },
        "nested2": { "type": "long" }
      }
    }
  }
}
```

This was causing ES to ignore the nested properties and create an empty nested type.