copy_worksheet failes on files with merged cells
*Created originally on Bitbucket by [steven somersbury (Liam Sanders)](https://bitbucket.org/%7B026e1b96-49c2-4f9b-96ab-4e2200c83817%7D/)*
**Env:** Python 3.8.1 with openpyxl 3.0.4 installed.
There seems to be some internal difference between programatically created merged cells and merged cells loaded from a file when copying worksheets
```python
from openpyxl import Workbook, load_workbook
# This works fine
wb = Workbook()
ws = wb.active
ws.merge_cells("A1:E1")
wb.copy_worksheet(ws)
wb.save("Result.xlsx")
# This breaks with a min_col error
wb = load_workbook("TestMergedCopy.xlsx")
newSheet = wb.copy_worksheet(wb["Sheet1"])
wb.save("Result2.xlsx")
```
”TestMergedCopy.xlsx” is a empty file with cells A1:E1 merged.
issue