df.assign with boolean dummy series results in NaN Series.
#### Code Sample, a copy-pastable example if possible

```python
import pandas
import io

csv_file = io.StringIO("""datetime,temperature_husconet,lat_husconet,lon_husconet,temperature_pws,dewpoint_pws,humidity_pws,lat_pws,lon_pws,temperature_eddh,dewpoint_eddh,windspeed_eddh,windgust_eddh,winddirection_eddh,pressure_eddh,humidity_eddh,precipitation_eddh,cloudcover_eddh_BKN,cloudcover_eddh_CAVOC,cloudcover_eddh_FEW,cloudcover_eddh_NSC,cloudcover_eddh_OVC,cloudcover_eddh_SCT,cloudcover_eddh_VV
2016-01-01 00:00:00,5.688,53.551105624479334,9.984235400036294,2.9,1.9,93.0,53.57898,9.79428,3.0,3.0,7.2,0.0,160.0,1023.0,94.0,,0,1,0,0,0,0,0
2016-01-01 00:00:00,4.508,53.540879976582616,9.995115621037971,2.9,1.9,93.0,53.57898,9.79428,3.0,3.0,7.2,0.0,160.0,1023.0,94.0,,0,1,0,0,0,0,0
2016-01-01 00:02:00,5.694,53.551105624479334,9.984235400036294,2.6,2.4,99.0,53.570890000000006,9.88888,3.0,3.0,7.2,0.0,160.0,1023.0,94.0,,0,1,0,0,0,0,0
2016-01-01 00:02:00,4.367,53.540879976582616,9.995115621037971,2.6,2.4,99.0,53.570890000000006,9.88888,3.0,3.0,7.2,0.0,160.0,1023.0,94.0,,0,1,0,0,0,0,0
""")

data_df = pandas.read_csv(
    csv_file,
    index_col="datetime",
    parse_dates=["datetime"]
)

df_hour = pandas.get_dummies(data_df.index.hour, prefix="hour")

df_hour_dict_version = {column: df_hour[column] for column in df_hour.columns}
data_df = data_df.assign(**df_hour_dict_version)

```
#### Problem description

I want to add the dummy variables back to the data frame as later in the process I will remove the index. 
While it is a dictionary, meaning `df_hour_dict_version`, everything is fine, the dummy columns are series.
They are uint8 values even though as dummy variables boolean would be more appropriate but that is how `get_dummies` is implemented.
But for some reason, after the assign the series turn into NaN columns. Out of the sudden all 0's and 1's are replaced by NaN!

#### Expected Output

Keep the series as they are.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.0.final.0
python-bits: 32
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.20.1
pytest: None
pip: 9.0.1
setuptools: 28.8.0
Cython: 0.26
numpy: 1.12.1
scipy: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>
