DataFrame.resample week with DST
#### Code Sample

```python
import pandas
import pytz

dataframe_1 = pandas.DataFrame(index=pandas.DatetimeIndex(
    ['2017-03-04 00:00', '2017-03-05 00:00', '2017-03-06 00:00', '2017-03-07 00:00', '2017-03-08 00:00'],
    tz=pytz.timezone('Europe/Amsterdam')),
    data=[11, 12, 13, 14, 15])
resampled_1 = dataframe_1.resample('1W').sum()
print(resampled_1)

dataframe_2 = pandas.DataFrame(index=pandas.DatetimeIndex(
    ['2017-03-25 00:00', '2017-03-26 00:00', '2017-03-27 00:00', '2017-03-28 00:00', '2017-03-29 00:00'],
    tz=pytz.timezone('Europe/Amsterdam')),
    data=[11, 12, 13, 14, 15])
resampled_2 = dataframe_2.resample('1W').sum()
print(resampled_2)

dataframe_3 = pandas.DataFrame(index=pandas.DatetimeIndex(
    ['2017-04-15 00:00', '2017-04-16 00:00', '2017-04-17 00:00', '2017-04-18 00:00', '2017-04-19 00:00'],
    tz=pytz.timezone('Europe/Amsterdam')),
    data=[11, 12, 13, 14, 15])
resampled_3 = dataframe_3.resample('1W').sum()
print(resampled_3)
```
#### Problem description

It looks like there is something wrong with _pandas.DataFrame.resample_ (by week) in relation to DST. 
When I run the above code sample I expect three times the same output, but the DST starting week 
 (dataframe_2 ) differ from the regular weeks.

#### Actual Output
```
                            0
2017-03-05 00:00:00+01:00  23
2017-03-12 00:00:00+01:00  42
                            0
2017-03-26 00:00:00+01:00  36
2017-04-02 00:00:00+02:00  29
                            0
2017-04-16 00:00:00+02:00  23
2017-04-23 00:00:00+02:00  42
```
#### Expected Output
```
                            0
2017-03-05 00:00:00+01:00  23
2017-03-12 00:00:00+01:00  42
                            0
2017-03-26 00:00:00+01:00  23
2017-04-02 00:00:00+02:00  42
                            0
2017-04-16 00:00:00+02:00  23
2017-04-23 00:00:00+02:00  42
```

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

<details>
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 16.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: None
LOCALE: en_US.UTF-8

pandas: 0.23.1
pytest: None
pip: 10.0.0
setuptools: 28.8.0
Cython: None
numpy: 1.14.1
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: 1.0.2
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>
