MultiIndex.tostring() produces empty string for non-empty index
With pandas 0.14.1, the following code produces an empty string:

```
import pandas
idx = pandas.MultiIndex(levels=[['a'],[0,1,2,3,4,4]], 
                        labels=[[0,0,0,0,0],[0,1,2,3,4]],
                        names=[0,1])
print idx.tostring()
```

Is this behavior correct? `Index.tostring()` appears to produce a non-empty string for a non-empty index.
