Option to suppress automatic conversion of tuples to MultiIndex 
Right now we have:

```
>>> pd.DataFrame({(1,2):pd.Series([2.3])}).columns
MultiIndex(levels=[[1], [2]],                     
           labels=[[0], [0]])                     
>>> pd.Index([(1,2)])
MultiIndex(levels=[[1], [2]],
           labels=[[0], [0]])
```

Could we have an option to suppress this behavior? One problem this causes is that the `.rename` method is not uniform and does not work if a tuple is silently converted:

```
>>> pd.DataFrame({(1,2):pd.Series([2.3])}).rename(columns={(1,2):(1,3)})
     1                                                                  
     2                                                                  
0  2.3                                                                  
```

To see why such behavior is problematic consider the following unintuitive example:

```
>>> a = ('a', 1)

>>> b = ('b', 2)

>>> pd.DataFrame({a:pd.Series([2,3])})
   a
   1
0  2
1  3

>>> pd.DataFrame({a:pd.Series([2,3])}).rename(columns={a:b})
   a
   1
0  2
1  3
```
