Comments are broken in write_only mode
*Created originally on Bitbucket by [markcho (Mark Cho)](https://bitbucket.org/%7Bad758203-258d-4b27-b8c5-6c6c8ffc4499%7D/)* Comments are broken on Workbook(write_only=True) To reproduce: ``` #!python wb = Workbook(write_only=True) ws = wb.create_sheet() cell = WriteOnlyCell(ws, 'Test 1') cell.comment = Comment(text='Hello World', author='openpyxl') headers = [cell] ws.append(headers) wb.save('test.xlsx') ``` This will save text.xlsx file but when I tried to open the file in Microsoft Excel, I get the following error (see attached image). Log file from Excel: ``` #!python <?xml version="1.0" encoding="UTF-8" standalone="yes"?> <recoveryLog xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"><logFileName>Repair Result to test 09449.xml</logFileName><summary>Errors were detected in file 'Macintosh HD:Users:...:test.xlsx'</summary><removedParts summary="Following is a list of removed parts:"><removedPart>Removed Part: Drawing shape.</removedPart></removedParts></recoveryLog> ``` *Attachments:* [Screen_Shot_2016-04-02_at_4.56.37_PM](/uploads/2f47ba4003effe7b2138027950fce896/Screen_Shot_2016-04-02_at_4.56.37_PM.png)
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