Fix missing CompleteMultipartUpload Etag. (#3227)

Fixes #3224

cmd/object-utils_test.go

@@ -158,7 +158,7 @@ func TestGetCompleteMultipartMD5(t *testing.T) {
 	}
 
 	for i, test := range testCases {
-		result, err := getCompleteMultipartMD5(test.parts...)
+		result, err := getCompleteMultipartMD5(test.parts)
 		if result != test.expectedResult {
 			t.Fatalf("test %d failed: expected: result=%v, got=%v", i+1, test.expectedResult, result)
 		}