Reindex with Series with name field drops name

```
In [34]: df2.pivot('Date', 'Individual')
Out[34]: 
            Exercises    
Individual          1   2
Date                     
1/1/2001           10 NaN
1/3/2001          NaN   5

In [35]: df2.pivot('Date', 'Individual').reindex(df['Date'])
Out[35]: 
            Exercises    
Individual          1   2
1/1/2001           10 NaN
1/2/2001          NaN NaN
1/3/2001          NaN   5
```
