Applying fill to cell with a NamedStyle applies fill to all cells with NamedStyle
*Created originally on Bitbucket by [AlainPannetier](https://bitbucket.org/%7B81b5a938-f5ad-4d7e-9253-3bf90c5f3df5%7D/)*
Simple test:
```
#!python
#!/usr/bin/env python3
from openpyxl import Workbook
from openpyxl.styles import NamedStyle, PatternFill, fills
myStyle = NamedStyle(name="myStyle")
wb = Workbook()
ws = wb.active
ws['A1'].value = 'A1'
ws['A1'].style = myStyle
ws['A2'].value = 'A2'
ws['A2'].style = myStyle
ws['A1'].fill = PatternFill(fill_type=fills.FILL_SOLID, fgColor='ff0000')
# Result ==> A2 has red background too
wb.save("test.xlsx")
```
*Attachments:* [test.xlsx](/uploads/2aa45ce8a1daa8a74e0e9b0d5a31cd69/test.xlsx)[test01.py](/uploads/5c2e32529b53ae73bf02e6fbffa9a581/test01.py)
issue