`PATCH /repos/{owner}/{repo}/milestones/{id}` always overwrote the due
date with `due_on`, so a request that only changed other fields, such as
`{"state": "closed"}`, silently removed it.
The due date is now kept when `due_on` is omitted, and `"due_on": null`
or `"due_on": ""` remove it.
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>