Incorrect _type serialization in statement render

[puerco]

When rendering statements as json, there is an artifact where _type is being encoded as "type" in the json output

{
  "predicateType": "https://in-toto.io/attestation/release/v0.1",
  "predicate": {
    "ownerId": "3935899",
    "purl": "pkg:github/puerco/test-immutable-releases@v1.0",
    "releaseId": "253765772",
    "repository": "puerco/test-immutable-releases",
    "repositoryId": "1073967940",
    "tag": "v1.0"
  },
  "_type": "",
  "type": "https://in-toto.io/Statement/v1",
  "subject": [
    {
      "uri": "pkg:github/puerco/test-immutable-releases@v1.0",
      "digest": {
        "sha1": "3ede92d1d86076be3e238618b5a54c8189668e3f"
      }
    }
  ]
}