Updating bucket name to the same name results in error

There was, probably, a recent change in influxdb since one of the litmus update buckets tests started failing. Before, the update was successful. influxdb@master a1ed6e3cecc16fa1e931dc97d80f43b28faabf40
If it is a change in functionality, please let me know and I will update the test.

- Create an org:
```
requests.post('http://localhost:9999/api/v2/orgs', data {"name":"test_org_name_already_exists","status:":"active"}
```

- create a bucket for the above org:
```
requests.post('http://localhost:9999/api/v2/buckets, data={"name":"test_bucket_name_already_exists", "retentionRules": [{"type":"expire","everySeconds":3600}], "organizationID": "03828e1d49a26000"})
```

- update created bucket with the same name:
```
requests.post('http://localhost:9999/api/v2/buckets/03828e1d4b226000, data={"name":"test_bucket_name_already_exists", "retentionRules": [{"type":"expire","everySeconds":3600}]})
```

- Request will return an error:
```
{
"message": "bucket name is not unique",
"code": "conflict"
}
```

Reproduce the bucket API sequence against the referenced InfluxDB commit and current master, focusing on updating a bucket without changing its name. Trace the bucket update and name-uniqueness validation path; done means the behavior is clarified and the regression is fixed or the test expectation is updated accordingly.

Written by the indexing model from the issue text.