Checking that a pandas.Series.index contains a value
#### Code Sample, a copy-pastable example if possible

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

some_numbers = np.random.randint(0,4,size=10)
print(some_numbers)
s = pd.Series(some_numbers)
gb = s.groupby(s).size() / len(s)
print(gb)
1.3 in gb
```
#### Problem description
I reported it [here](https://stackoverflow.com/questions/51563433/checking-that-a-pandas-series-index-contains-a-value). The output of the last line is True instead of being False.

#### Expected Output
False

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
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: 0.23.3
pytest: 3.2.1
pip: 9.0.1
setuptools: 40.0.0
Cython: 0.26.1
numpy: 1.15.0
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.2.2
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 0.9.8
lxml: 3.8.0
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
