common/hobject: fix hobject_t::to_str()

common/hobject: fix hobject_t::to_str()

Introduced in: e462f76aedbb89e8db57dc2324d4f5e9fe54cf9e
Causing the following failures:
```
[  FAILED  ] SnapMapperTest.CheckObjectKeyFormat
[  FAILED  ] SnapMapperTest.CheckRawKeyFormat
```
with the following error:
```
2024-03-09T17:31:57.117 INFO:teuthology.orchestra.run.smithi060.stdout:Expected equality of these values:
2024-03-09T17:31:57.117 INFO:teuthology.orchestra.run.smithi060.stdout:  object_key
2024-03-09T17:31:57.117 INFO:teuthology.orchestra.run.smithi060.stdout:    Which is: "OBJ_.1_7FFFFFFFFFFFFFEB.76543210.FFFFFFFFFFFFFFEC.test%uobject..namespace"
2024-03-09T17:31:57.117 INFO:teuthology.orchestra.run.smithi060.stdout:  test_object_key
2024-03-09T17:31:57.117 INFO:teuthology.orchestra.run.smithi060.stdout:    Which is: "OBJ_.1_7FFFFFFFFFFFFFEB.76543210.ffffffffffffffec.test%uobject..namespace"
```

Signed-off-by: Matan Breizman <mbreizma@redhat.com>