Export to excel for multiindex columns
Hi All,

I'm just using 0.17 for testing exporting multiindex column dataframe. When i'm pivot table dataframe with X column as index (and without "columns" argument), **which will not create multiindex column**, 

pivot table code (without columns argument):

```
df.pivot_table(index=["X"], columns=[], aggfunc={"Y": len,"Z":np.sum}, fill_value=0)
df.to_excel(FILE_NAME)
```

the output using to_excel() follow new format:

![New format output](http://pandas.pydata.org/pandas-docs/version/0.17.0/_images/new-excel-index.png)

But, the question arise when i'm trying to pivot table X column as index (and with "columns" argument), **which will create multiindex columns**. Then, i'm trying to export it using to_excel() and still follow old format.

pivot table code (with columns argument):

```
df.pivot_table(index=["X"], columns=["A"], aggfunc={"Y": len,"Z":np.sum}, fill_value=0)
df.to_excel(FILE_NAME)
```

![Old format output](http://pandas.pydata.org/pandas-docs/version/0.17.0/_images/old-excel-index.png)

I don't know if this newest excel export format only applied to single column index, not multicolumn index. Was this case expected or there is something wrong?

Thanks
