convert_objects(convert_dates='coerce') is not coercing to date
Shouldn't `convert_objects(convert_dates='coerce')` be coercing a series of strings to NaTs?

``` python
df = pd.DataFrame({'date': ['a', 'b', 'c', 'd']})
df.convert_objects(convert_dates='coerce')
  date
0    a
1    b
2    c
3    d

```
