pd.date_range AssertionError: Inferred time zone not equal to passed time zone
#### Code Sample, a copy-pastable example if possible

```python
pd.date_range(start='2017-06-01 00:00:00+02:00',
              end='2017-08-01 00:00:00+02:00',
              freq='15min',
              tz='Europe/Berlin')
```
```
AssertionError: Inferred time zone not equal to passed time zone
```
#### Problem description

The given times are valid times in the `Europe/Berlin` timezone. However, as can be seen when the `tz` parameter is not passed, pandas detects them as `pytz.FixedOffset(120)` and then raises an error because this is not the same as `Europe/Berlin`.

When giving tz-aware datetimes with different UTC offsets due to a daylight savings change (e.g. `2017-06-01 00:00:00+02:00` and `2017-12-01 00:00:00+01:00`), there is a 
```
TypeError: Start and end cannot both be tz-aware with different timezones
```

#### Expected Output
```
DatetimeIndex(['2017-06-01 00:00:00+02:00', '2017-06-01 00:15:00+02:00',
               '2017-06-01 00:30:00+02:00', '2017-06-01 00:45:00+02:00',
               ...
               '2017-07-31 23:30:00+02:00', '2017-07-31 23:45:00+02:00'],
              dtype='datetime64[ns, Europe/Berlin]', length=6048, freq='15T')
```

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

<details>

pd.show_versions()
/home/cjk/.conda/lib/python3.6/site-packages/xarray/core/formatting.py:16: FutureWarning: The pandas.tslib module is deprecated and will be removed in a future version.
  from pandas.tslib import OutOfBoundsDatetime

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.10.0-27-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.20.1
pytest: 3.0.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: 0.9.2
IPython: 5.3.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.3.0
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>
