Wrong start datetime for data_range
#### Code Sample, a copy-pastable example if possible

```python
>>>pd.date_range(start='1/1/2018', periods=5, freq='M')
DatetimeIndex(['2018-01-31', '2018-02-28', '2018-03-31', '2018-04-30',
               '2018-05-31'],
              dtype='datetime64[ns]', freq='M')
```
#### Problem description

Why datatime start from '2018-01-31' ?