UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-1: ordinal not in range(128)
*Created originally on Bitbucket by [jefffffrey](https://bitbucket.org/%7B570e58b5-aa20-474f-80d8-69080c421a61%7D/)*
```
#!python
File "/home/jeffrey/workspace/romulan-env/local/lib/python2.7/site-packages/openpyxl/workbook/workbook.py", line 345, in save
save_workbook(self, filename)
File "/home/jeffrey/workspace/romulan-env/local/lib/python2.7/site-packages/openpyxl/writer/excel.py", line 266, in save_workbook
writer.save(filename)
File "/home/jeffrey/workspace/romulan-env/local/lib/python2.7/site-packages/openpyxl/writer/excel.py", line 248, in save
self.write_data()
File "/home/jeffrey/workspace/romulan-env/local/lib/python2.7/site-packages/openpyxl/writer/excel.py", line 81, in write_data
self._write_worksheets()
File "/home/jeffrey/workspace/romulan-env/local/lib/python2.7/site-packages/openpyxl/writer/excel.py", line 197, in _write_worksheets
xml = ws._write()
File "/home/jeffrey/workspace/romulan-env/local/lib/python2.7/site-packages/openpyxl/worksheet/worksheet.py", line 870, in _write
return write_worksheet(self)
File "/home/jeffrey/workspace/romulan-env/local/lib/python2.7/site-packages/openpyxl/writer/worksheet.py", line 163, in write_worksheet
tables = _add_table_headers(ws)
File "/home/jeffrey/workspace/romulan-env/local/lib/python2.7/site-packages/openpyxl/writer/worksheet.py", line 188, in _add_table_headers
col.name = str(cell.value)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-1: ordinal not in range(128)
```
when I write some non-ascii char into excel, the error raised.
then I change `col.name = str(cell.value)` to`col.name = cell.value`, it's ok.
issue