Fix issue 6051

Bug: `mangle_dupe_cols` cannot work while the header is a list. 

Originally, `has_mi_columns` will be set as 1 while the header is a list. And the `mangle_dupe_cols` things would not work when `has_mi_columns` == 1. 

This pull request is to 

1. for the list with single element, for example [0], the `has_mi_columns` will be set as 0, and the header will do the original `mangle_dupe_cols` things as the header is a single integer.
2. for the list with more than one element, append the sequence number in the last element of the duplicated multi-column

For example

    Male,Male,Male,Male,Male,Female,Female
    A,B,B,B,C,D,D
    1,2,3,4,5,6,7
    1,2,3,4,5,6,7
    1,2,3,4,5,6,7
    1,2,3,4,5,6,7
    1,2,3,4,5,6,7
    1,2,3,4,5,6,7

would be converted to 

    In [2]: pd.read_csv('woo.csv', header=[0,1])
    Out[2]: 
       Male                  Female     
          A  B  B.1  B.2  C       D  D.1
    0     1  2    3    4  5       6    7
    1     1  2    3    4  5       6    7
    2     1  2    3    4  5       6    7
    3     1  2    3    4  5       6    7
    4     1  2    3    4  5       6    7
    5     1  2    3    4  5       6    7

    [6 rows x 7 columns]