[BUG] `cudf.date_range` doesn't match pandas for weekly frequencies

**Describe the bug**
See repro

**Steps/Code to reproduce bug**
cudf:
```python
import cudf
cudf.date_range('2016-01-01 01:01:00', periods=5, freq='W', tz=None)
```
```
DatetimeIndex(['2016-01-01 01:01:00', '2016-01-08 01:01:00',
'2016-01-15 01:01:00', '2016-01-22 01:01:00',
'2016-01-29 01:01:00'],
dtype='datetime64[ns]', freq='7D')
```
pandas:
```python
import pandas as pd
pd.date_range('2016-01-01 01:01:00', periods=5, freq='W', tz=None)
```
```
DatetimeIndex(['2016-01-03 01:01:00', '2016-01-10 01:01:00',
'2016-01-17 01:01:00', '2016-01-24 01:01:00',
'2016-01-31 01:01:00'],
dtype='datetime64[ns]', freq='W-SUN')
```
**Expected behavior**
Match pandas