Cell style name remains same even after NamedStyle assigned to cell
*Created originally on Bitbucket by [asovetnikov (Artem Sovetnikov)](https://bitbucket.org/%7B0b47aa0f-ddc6-4ae2-bb0e-aaa073bf9e67%7D/)*
This code produces "Normal Normal Normal" (three times Normal).
Expected result is "Normal style1 style1"
empty.xlsx is empty spreadsheet from Excel.
```
#!python
from openpyxl import load_workbook
from openpyxl.styles import NamedStyle
wb = load_workbook('q:\\temp\\empty.xlsx')
cell = wb.worksheets[0].cell(row=1,column=1)
print(cell.style)
ns = NamedStyle(name='style1')
cell.style = ns
print(cell.style)
cell.style = ns.name
print(cell.style)
```
Seems named style xfId is lost when StyleArray copy made in last line of NamedStyleDescriptor.__set__
This behavior prevents style copy from cell to cell just by source cell style name.
issue