Conditional formatting not saved when using write-only mode
*Created originally on Bitbucket by [Cristian Ciupitu](https://bitbucket.org/%7Be4cdd06f-2927-4afe-9e66-05667b856116%7D/)*
If I use conditional formatting rules in the optimized write only mode they aren't saved.
```
#!python
from openpyxl import Workbook
from openpyxl.formatting.rule import CellIsRule
from openpyxl.styles import PatternFill
redFill = PatternFill(start_color='EE1111', end_color='EE1111',
fill_type='solid')
wb = Workbook(write_only=True)
ws = wb.create_sheet(title='Conditional')
ws.conditional_formatting.add(
'D2:D10',
CellIsRule(
operator='between', formula=['1', '5'], stopIfTrue=True, fill=redFill
)
)
wb.save('test_write-only.xlsx')
```
issue