Data corruption during Excel export
In some case a DataFrame exported to excel present some bad values.
It's is not a problem of Excel reading (the data inside the sheet1.xml of the .xlsx file is also incorrect).

The same DataFrame exported to ".csv" is correct.

The problem could be "solved" by renaming the column header  as [col-1, col-2,...]. Maybe an encoding problem ?

The issue is that there is no warning/error during the export. It's very easy to miss it.

To reproduce:

```
import pandas as pd

df = pd.read_pickle('problematic_df.pkl')
df.to_excel('problematic_df.xlsx')
df.to_csv('problematic_df.csv')
```

with the file available here: https://drive.google.com/file/d/0Bzz_ZaP_wS_HMFdlMkVzaTR0cjA/view?usp=sharing 

Note that the content of cell M14 is different in both file (at least when run on my computer)

Using: 
- Python 3.4.3 |Anaconda 2.3.0 (64-bit)
- pandas 0.16.2
- Windows 7 64 bits
