fix: report the last upload in ListMultipartUploads next markers on a single page

NextKeyMarker and NextUploadIdMarker were only set for a truncated
listing, so a ListMultipartUploads response that fit in one page
returned them empty. S3 still sets them to the key and upload ID of
the last upload in the response.

Fill both markers from the last listed upload when the listing is not
truncated, including the case where the page ends exactly at
max-uploads. A page of common prefixes only keeps them empty, as
before. Enable the REST test that covers it.

Fixes #2144

tests/integration/group-tests.go

@@ -542,6 +542,7 @@ func TestListMultipartUploads(ts *TestState) {
 	ts.Run(ListMultipartUploads_empty_result)
 	ts.Run(ListMultipartUploads_invalid_max_uploads)
 	ts.Run(ListMultipartUploads_max_uploads)
+	ts.Run(ListMultipartUploads_next_markers_not_truncated)
 	ts.Run(ListMultipartUploads_exceeding_max_uploads)
 	ts.Run(ListMultipartUploads_ignore_upload_id_marker)
 	ts.Run(ListMultipartUploads_invalid_uploadId_marker)
@@ -3250,6 +3251,7 @@ func GetIntTests() IntTests {
 		"ListMultipartUploads_empty_result":                                                ListMultipartUploads_empty_result,
 		"ListMultipartUploads_invalid_max_uploads":                                         ListMultipartUploads_invalid_max_uploads,
 		"ListMultipartUploads_max_uploads":                                                 ListMultipartUploads_max_uploads,
+		"ListMultipartUploads_next_markers_not_truncated":                                  ListMultipartUploads_next_markers_not_truncated,
 		"ListMultipartUploads_exceeding_max_uploads":                                       ListMultipartUploads_exceeding_max_uploads,
 		"ListMultipartUploads_ignore_upload_id_marker":                                     ListMultipartUploads_ignore_upload_id_marker,
 		"ListMultipartUploads_invalid_uploadId_marker":                                     ListMultipartUploads_invalid_uploadId_marker,