Retry.with_delay sets multiplier even when it's None

#### Steps to reproduce

```python
from google.api_core import retry

retry.Retry(predicate=...).with_delay(initial=0.5, maximum=5 * 60)
```
`retry._multiplier` will be set to None because `with_delay` sets it when `maximum` is given


PR: https://github.com/googleapis/python-api-core/pull/164