Cannot save workbook with diagonal border.
*Created originally on Bitbucket by [panda-34](https://bitbucket.org/%7B118b34a6-98c8-40a5-9596-e565558636f8%7D/)* I create a new file in Excel 2013, make a diagonal border on a cell and save the file. openpyxl loads the workbook, but when saving it the error is produced: File "C:\Lib\Python\lib\xml\etree\ElementTree.py", line 1081, in _escape_attrib if "&" in text: TypeError: argument of type 'bool' is not iterable. The error happens when trying to serialize xl / styles.xml / styleSheet / borders / border / "diagonalDown" attribute which has value True, though in original file it has value "1". python 3.4, 3.5 openpyxl 2.3, 2.4 *Attachments:* [diagonal.xlsx](/uploads/1409a7ca4358696757f869db7d5d3a9c/diagonal.xlsx)
issue