numpy.datetime64 casted to Timestamp when added to DataFrame
```python
import numpy
import pandas as pd

# We create a list of strings. 
time_str_arr = ['2017-06-30T13:51:15.854', '2017-06-30T13:51:16.250',
                '2017-06-30T13:51:16.452', '2017-06-30T13:51:16.659']
# Then we create a time array, rounded to 10ms (actually floored, 
# not rounded), everything seems to be fine here.
rounded_time = numpy.array(time_str_arr, dtype="datetime64[10ms]")
rounded_time 

# Then we create a Pandas DataFrame and assign the time array as a 
# column to it. The datetime64 content is destroyed.
d = {'one' : pd.Series([1., 2., 3.], index=['a', 'b', 'c']),
  'two' : pd.Series([1., 2., 3., 4.], index=['a', 'b', 'c', 'd'])}
df = pd.DataFrame(d)
df = df.assign(wrong_time=rounded_time)
df

# The output I get:
#    one two wrong_time
# a   1.0 1.0 1974-10-01 18:11:07.585
# b   2.0 2.0 1974-10-01 18:11:07.625
# c   3.0 3.0 1974-10-01 18:11:07.645
# d   NaN 4.0 1974-10-01 18:11:07.665

# (the timestamps are wrong)
```
#### Problem description

I am having a list of timestamps, with millisecond accuracy, encoded as strings. Then I round them to 10ms resolution, that goes well. The bug comes when I add the rounded timestamps to DataFrame as a new column - the values of datetime64 objects get totally destroyed. My suspicion - the numpy.datetime64 is converted to some other datatype in the DataFrame.assign() method. It should maintain the same type.

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

<details>

INSTALLED VERSIONS
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

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: None
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: 3.2.2
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>
