Unable to save file from workbook based on file that contains colored tabs
*Created originally on Bitbucket by [mperreux (Michael Perreux)](https://bitbucket.org/%7B2f264c60-af97-4edf-a52a-26345a4aadf0%7D/)* Reproducing the issue: Create xlsx with colored tabs save as hello.xlsx. Run following code: ``` #!python from openpyxl import load_workbook wb = load_workbook("hello.xlsx") wb.save("hello2.xlsx") ``` ``` #!python Traceback (most recent call last): File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/xml/etree/ElementTree.py", line 1081, in _escape_attrib if "&" in text: TypeError: argument of type 'int' is not iterable During handling of the above exception, another exception occurred: File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/xml/etree/ElementTree.py", line 1059, in _raise_serialization_error "cannot serialize %r (type %s)" % (text, type(text).__name__) TypeError: cannot serialize 7 (type int) ``` *Attachments:* [color_bug.xlsx](/uploads/4e158ed3b70b0bf41faad416d0ee1c6d/color_bug.xlsx)[color_bug_screen.xlsx](/uploads/876a5e11e48bd95e067c6994c20cb090/color_bug_screen.xlsx.png)
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