Unable to save file from workbook based on file that contains colored tabs

Created originally on Bitbucket by mperreux (Michael Perreux)

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.xlsxcolor_bug_screen.xlsx