"NA" value from Excel read by Pandas as nan
When doing a:
df = pd.read_excel('cb2.xlsx') in the column 'Zone', in which there are strings like "APAC", "EUR", "NA"
Surprisingly, the value "NA" is interpreted by pandas as nan

[cb2.xlsx](https://github.com/pydata/pandas/files/61735/cb2.xlsx)
