ContentHubEntityExportController sets Content-Type request header, yet there is no request body

\Drupal\acquia_contenthub\Controller\ContentHubEntityExportController::getEntityCdfByInternalRequest() sets the Content-Type request header, but that header only makes sense when there is a request body.

AFAICT it's only being set because \Drupal\acquia_contenthub\Client\ClientManager::getRequestSignature() is called later and needs that header to be set. Which would suggest this is a code smell.