columns sometimes not added to empty dataframe
#### Code Sample, a copy-pastable example if possible

```python
import datetime
import pandas as pd

test = pd.DataFrame({'date':[datetime.datetime(2000,1,1)]}).set_index('date')
test = test[0:0].copy()

test['3010'] = None
test['2010'] = None

print(test)
```
#### Problem description

The column '3010' is added to the dataframe as expected, but column 2010 does not get added.

#### Expected Output

Empty DataFrame
Columns: [3010, 2010]
Index: []

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.5.4.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 85 Stepping 4, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 0.25.1
numpy            : 1.17.2
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 9.0.1
setuptools       : 36.4.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
openpyxl         : None
odfpy            : None
gcsfs            : None
bottleneck       : None
pandas_gbq       : None
xarray           : None
tables           : None
scipy            : None
pyarrow          : None
xlsxwriter       : None
matplotlib       : None
sqlalchemy       : None
pytables         : None
xlwt             : None
lxml.etree       : None
s3fs             : None
xlrd             : None
numexpr          : None
bs4              : None
fastparquet      : None

</details>
