Troubles with date_range and DST
#### Code Sample

```python
>>> pd.date_range('2018-10-23', '2018-11-06', freq='7D', tz='Europe/Paris')
DatetimeIndex(['2018-10-23 00:00:00+02:00', '2018-10-29 23:00:00+01:00', '2018-11-05 23:00:00+01:00'], dtype='datetime64[ns, Europe/Paris]', freq='7D')

>>> pd.date_range('2018-03-25', '2018-04-09', freq='7D', tz='Europe/Paris')
DatetimeIndex(['2018-03-25 00:00:00+01:00', '2018-04-01 01:00:00+02:00', '2018-04-08 01:00:00+02:00'], dtype='datetime64[ns, Europe/Paris]', freq='7D')
```
#### Problem description

It seems that the `D` frequency is somewhat broken around DST dates. The code above gives the same result as `pd.date_range(..., freq='168H', tz='Europe/Paris')`, but it should be different.

#### Expected Output

```python
>>> pd.date_range('2018-10-23', '2018-11-06', freq='7D', tz='Europe/Paris')
DatetimeIndex(['2018-10-23 00:00:00+02:00', '2018-10-30 00:00:00+01:00', '2018-11-06 00:00:00+01:00'], dtype='datetime64[ns, Europe/Paris]', freq='7D')

>>> pd.date_range('2018-03-25', '2018-04-09', freq='7D', tz='Europe/Paris')
DatetimeIndex(['2018-03-25 00:00:00+01:00', '2018-04-01 00:00:00+02:00', '2018-04-08 00:00:00+02:00'], dtype='datetime64[ns, Europe/Paris]', freq='7D')
```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 3.16.0-5-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: fr_FR.UTF-8
LOCALE: fr_FR.UTF-8

pandas: 0.22.0
pytest: None
pip: 9.0.3
setuptools: 39.0.1
Cython: None
numpy: 1.14.2
scipy: 1.0.1
pyarrow: None
xarray: None
IPython: 6.3.0
sphinx: None
patsy: None
dateutil: 2.7.2
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
