BUG: in read_csv, keep_date_cols doesn't result in correct dtype
`>>> import pandas`
`>>> import io`
`>>> data = """A`
`20150908`
`20150909`
`"""`
`>>> t=pandas.read_csv(io.StringIO(data))`
`>>> t.dtypes`
`A    int64`
`dtype: object`
`>>> t=pandas.read_csv(io.StringIO(data),parse_dates={'date':['A']},keep_date_col=True)`
`>>> t.dtypes`
`date    datetime64[ns]`
`A               object`
`dtype: object`

The second time, the datatype of the column 'A' should also be int64.
