Value in column with only 1 row returns False instead of True
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
import pandas as pd

c = pd.DataFrame([[1,2],], columns = ['c1', 'c2'])
1 in c.c1

new_row = {'c1':3,
'c2':4}
c = c.append(new_row, ignore_index=True)
1 in c.c1

observed output:
In [282]: c = pd.DataFrame([[1,2],], columns = ['c1', 'c2'])
     ...: 1 in c.c1
     ...:
Out[282]: False

In [283]: new_row = {'c1':3,
     ...:             'c2':4}
     ...: c = c.append(new_row, ignore_index=True)
     ...: 1 in c.c1
     ...:
Out[283]: True

```
#### Problem description
Checking for a value in a column reports False when there is only one row (which has that value).


#### Expected Output
If that column ( of a single value) has that value, it should return True


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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
In [284]: pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 62 Stepping 4, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.20.3
pytest: 3.2.1
pip: None
setuptools: 36.5.0.post20170921
Cython: 0.26.1
numpy: 1.13.3
scipy: 0.19.1
xarray: None
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.1.0
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.0
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
