Dropna Subnet changes timestamp format in to_csv()
#### Code Sample, a copy-pastable example if possible

```
import pandas as pd
import datetime

date_example_string = "1911180945"
ts = datetime.datetime.strptime(
    date_example_string, "%y%m%d%H%M%S"
)
test_json = [
    {
        "created_at": "2019-11-18 16:28:42.932887",
        "foo": "bar",
    }
]

df = pd.DataFrame(test_json)
df["baz"] = ts

print ("=== before ===")
print (df.to_csv())
df["created_at"] = pd.to_datetime(
    df["created_at"], infer_datetime_format=True, errors="coerce"
)

print("=== after ===")
print (df.to_csv())

print("=== dropna ===")
df = df.dropna(subset=["created_at"])
print(df.to_csv())

```
#### Problem description

When using pd.dropna, it changes the format of a datetime column to a different format when calling to_csv()

```
=== before ===
,created_at,foo,baz
0,2019-11-18 16:28:42.932887,bar,2019-11-18 09:04:05

=== after ===
,created_at,foo,baz
0,2019-11-18 16:28:42.932887,bar,2019-11-18 09:04:05

=== dropna ===
,created_at,foo,baz
0,2019-11-18 16:28:42.932887,bar,2019-11-18 09:04:05.000000

```

As you can see after calling dropna the format of the baz column is now 2019-11-18 09:04:05.000000

#### Expected Output

```
=== before ===
,created_at,foo,baz
0,2019-11-18 16:28:42.932887,bar,2019-11-18 09:04:05

=== after ===
,created_at,foo,baz
0,2019-11-18 16:28:42.932887,bar,2019-11-18 09:04:05

=== dropna ===
,created_at,foo,baz
0,2019-11-18 16:28:42.932887,bar,2019-11-18 09:04:05

```

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

<details>

>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.6.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : en_US.UTF-8

pandas           : 0.25.3
numpy            : 1.16.2
pytz             : 2018.9
dateutil         : 2.8.0
pip              : 19.0.3
setuptools       : 40.8.0
Cython           : 0.29.13
pytest           : 5.2.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8 (dt dec pq3 ext lo64)
jinja2           : 2.10.3
IPython          : 7.8.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None


</details>
