Converter returning string passed to read_csv causes exception

for example:

```
        data = """Id;Number1;Number2;Text1;Text2;Number3
1;1521,1541;187101,9543;ABC;poi;4,738797819
2;121,12;14897,76;DEF;uyt;0,377320872
3;878,158;108013,434;GHI;rez;2,735694704"""
        f = lambda x : x.replace(",", ".")
        converter = {'Number1':f,'Number2':f, 'Number3':f}
        df2 = read_csv(StringIO(data), sep=';',converters=converter)
```
