BUG: DataFrame.sort_values(by, ascending): by CANNOT recognize a tuple, yet ascending can. 
This behavior looks like a bug to me. 

Inconsistent behaviors of the argument for **DataFrame.sort_values**( )

The first argument takes list, but fail to take tuple .

These worked: 

``` python
   df.sort_values( ['A', 'B'], ascending= [1, 0] )  # list, list
   df.sort_values( ['A', 'B'], ascending= (1, 0) )  # list, tuple 
```
These failed: 

``` python
   df.sort_values( ('A', 'B'), ascending= [1, 0] )  # tuple, list
   df.sort_values( ('A', 'B'), ascending= (1, 0) )  # tuple, tuple 
```
Error:
``` phthon
raise ValueError(
ValueError: Length of ascending (2) != length of by (1)
```