adding duration created with object fail since 1.11.9

**Describe the bug**
dayjs().add(dayjs.duration({weeks: 1})) => today

**Expected behavior**
dayjs().add(dayjs.duration({weeks: 1})) => today + 7 days

**Information**
- Day.js Version [e.g. v1.11.13]
- Day.js Version 1.11.7 was OK

Start by reproducing the reported difference between Day.js 1.11.7 and the later version using dayjs().add(dayjs.duration({weeks: 1})). Trace the duration-object handling and identify the relevant existing tests, then confirm that a one-week duration advances the date by seven days without breaking other duration inputs.

Written by the indexing model from the issue text.