Redmine

When we copy a closed issue, the value of "closed_on" of copied issue is always set to source issue's value, even if the status of copied issue is not closed. I think it should be nil because the copied issue have not been closed before.

Steps to reproduce:

1. Create a closed issue (Issue_A).
2. Click "Copy" on Issue_A.
3. Change status to "New" and create a new copied issue (Issue_B).
4. "closed_on" field of Issue_B should be nil, but the value is same with Issue_A.