Merge cells after copying a worksheet change original worksheet.
*Created originally on Bitbucket by [bonapart71 (Andrew Nikolayenkov)](https://bitbucket.org/%7Baf06d5c5-62d3-4fe0-b4b4-d069e8f38adc%7D/)*
A small example:
```
#!python
from openpyxl import Workbook
wb = Workbook()
ws1 = wb.active
ws2 = wb.copy_worksheet(ws1)
ws2.merge_cells('A1:C3')
wb.save('test.xlsx')
```
In the file, you can see that the cells are merged in both sheets - ws1 and w2.
issue