read_csv not correctly parsing dates when parse_dates is string and index_col not set
With:

```
csv="""A,B,C
1,2,2003-11-1
"""
```

These all work as expected:

```
In [40]: pd.read_csv(StringIO(csv), parse_dates="C",index_col="C").index[0]

Out[40]: Timestamp('2003-11-01 00:00:00', tz=None)

In [41]: pd.read_csv(StringIO(csv), parse_dates=["C"],index_col="C").index[0]

Out[41]: Timestamp('2003-11-01 00:00:00', tz=None)

In [42]: pd.read_csv(StringIO(csv), parse_dates=["C"]).C[0]

Out[42]: Timestamp('2003-11-01 00:00:00', tz=None)
```

but this does not parse the string:

```
In [39]: pd.read_csv(StringIO(csv), parse_dates="C",).C[0]

Out[39]: '2003-11-1'
```
