`Workbook._named_styles['Normal'].number_format` behave strangely.
*Created originally on Bitbucket by [soundlake (Sinyeol An)](https://bitbucket.org/%7B76cc76f5-9ed0-461d-9b0e-bb0f56cd213e%7D/)*
### The sample code ###
```
#!python
from openpyxl import Workbook
from openpyxl.styles.numbers import FORMAT_TEXT
wb = Workbook()
wb._named_styles['Normal'].number_format = FORMAT_TEXT
wb.save('hello.xlsx')
```
### The expected behavior ###
- The "Normal" named style's number format is set to "@" (text format)
- All the cells' number format is set to "@" (text format)
### The current behavior ###
- [O] The "Normal" named style's number format is set to "@" (text format)
- [X] All the cells' number format is set to "@" (text format)
- It seems to break the relationship between all cells and the "Normal" style.
- If a cell or a range of cells are set to another style then re-set to this "Normal" style, then the cell's or cells' number format changes into "@" (text format).
- If you didn't try the previous procedure to re-connect cells with this "Normal" style, although the number format of "Normal" style changes, all the cells are not affected. (It should be, and it is in the Excel application.)
### More info ###
- This might be related with #920.
- I'm not sure exactly which component is related with this. It might be `workbook` or `writer` or even `styles`.
- I've been using Debian (stretch) package and it's only `2.3.x`. But I tested this with `2.5.x`, too, and got same result.
issue