append method breaks final xlsx file
*Created originally on Bitbucket by [kcaper (Oleksandr Romanets)](https://bitbucket.org/%7B37aa4983-ea54-4e58-80c3-8b88fc4c2522%7D/)*
```python
from openpyxl import load_workbook
import os
basedir = '/home/user/projects/test_openpyxl'
template = os.path.join(basedir, 'test.xlsx')
wb_t = load_workbook(template)
wall_sheet = wb_t[u'2.1']
r = [1, 2, 3, 4, 5]
wall_sheet.append(r)
wb_t.save(os.path.join(basedir, 'testfileafterrun.xlsx'))
```
When updating from version 2.5.12 to 3.0.0 the code above started to break the final Excel file.

In `worksheet.py` `Worksheet.append()` the last row of sheet is identified as 1048577, however indicated sheet has only 5 rows.

*Attachments:* [test.xlsx](/uploads/edd962b6e4ae4c44d9b7ffa7b78e4975/test.xlsx)
issue