Improve ndjson mock

[delvedor]

With this pr, any request with content-type: application/x-ndjson is handled correctly and given to the user as an array of objects.
Example:

const mock = new Mock()
const client = new Client({
  node: 'http://localhost:9200',
  Connection: mock.getConnection()
})

mock.add({
  method: 'POST',
  path: '/_bulk'
}, params => {
  assert.deepStrictEqual(params.body, [
    { foo: 'bar' },
    { baz: 'fa\nz' }
  ])
  return { status: 'ok' }
})

const response = await client.bulk({
  body: [
    { foo: 'bar' },
    { baz: 'fa\nz' }
  ]
})
console.log(response)