code: public Observable<BackupCertificateResult> backupCertificateAsync(String vaultBaseUrl, String certificateName) {
          return backupCertificateWithServiceResponseAsync(vaultBaseUrl, certificateName).map(new Func1<ServiceResponse<BackupCertificateResult>, BackupCertificateResult>() {
              @Override
              public BackupCertificateResult call(ServiceResponse<BackupCertificateResult> response) {
                  return response.body();
              }
          });
      }
docs: Backs up the specified certificate.
  Requests that a backup of the specified certificate be downloaded to the client. All versions of the certificate will be downloaded. This operation requires the certificates/backup permission.
  
  @param vaultBaseUrl The vault name, for example https://myvault.vault.azure.net.
  @param certificateName The name of the certificate.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the BackupCertificateResult object