read_csv fails when both converts and index_col are used on the same column
``` python
In [473]: d = """\
A;B
1;2
3;4
"""

In [474]: pandas.read_csv(StringIO(d), sep=';', converters={'A': lambda x: x}, index_col='A')
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-474-6bc08702ea91> in <module>()
...
KeyError: 'A'

In [475]: pandas.read_csv(StringIO(d), sep=';', converters={'A': lambda x: x}).set_index('A')
Out[475]: 
   B
A   
1  2
3  4
```
