Extended submission is merged with submitter

[matthew-white]

When a submission is requested with extended metadata, a submitter property is added to the response (the submission object). However, it looks like that submitter object is also merged with the submission object. The submission and the submitter share a couple of properties (createdAt and updatedAt), and as a result of the merge, it looks like the submission's properties are overwritten by the submitter's. For example, I added console.log(body) to the following test:

central-backend/test/integration/api/submissions.js

Lines 3169 to 3181 in 0057300

	it('should return with extended metadata if requested', testService((service) =>
	service.login('alice', (asAlice) =>
	asAlice.post('/v1/projects/1/forms/simple/submissions')
	.send(testData.instances.simple.one)
	.set('Content-Type', 'text/xml')
	.expect(200)
	.then(() => asAlice.get('/v1/projects/1/forms/simple/submissions/one')
	.set('X-Extended-Metadata', 'true')
	.expect(200)
	.then(({ body }) => {
	body.should.be.an.ExtendedSubmission();
	body.submitter.displayName.should.equal('Alice');
	})))));

The body looked like this:

{
  instanceId: 'one',
  submitterId: 5,
  deviceId: null,
  createdAt: '2023-05-05T12:45:20.261Z',
  updatedAt: null,
  reviewState: null,
  userAgent: 'node-superagent/3.8.3',
  currentVersion: {
    submitterId: 5,
    createdAt: '2023-05-05T12:45:20.886Z',
    instanceName: null,
    instanceId: 'one',
    current: true,
    deviceId: null,
    userAgent: 'node-superagent/3.8.3',
    submitter: {
      id: 5,
      type: 'user',
      displayName: 'Alice',
      createdAt: '2023-05-05T12:45:20.261Z',
      updatedAt: null,
      deletedAt: null
    }
  },
  submitter: {
    id: 5,
    type: 'user',
    displayName: 'Alice',
    createdAt: '2023-05-05T12:45:20.261Z',
    updatedAt: null,
    deletedAt: null
  },
  id: 5,
  type: 'user',
  displayName: 'Alice',
  deletedAt: null
}