@Test
public void start() {

  ArgumentCaptor<InitiateMultipartUploadRequest> request = ArgumentCaptor
      .forClass(InitiateMultipartUploadRequest.class);
  InitiateMultipartUploadResult response = mock(InitiateMultipartUploadResult.class);
  when(response.getUploadId()).thenReturn(UPLOAD_ID);
  when(s3.initiateMultipartUpload(request.capture())).thenReturn(response);
  String result = underTest.start();
  assertThat(result, is(UPLOAD_ID));
  assertThat(request.getValue().getBucketName(), is(BUCKET));
  assertThat(request.getValue().getKey(), is(KEY));
}