Change in column name creates an empty output for wide_to_long function
#### Code Sample, a copy-pastable example if possible

```python
df = pd.DataFrame({'person_id' :[1,2,3],'date1': 
['12/31/2007','11/25/2009','10/06/2005'],'val1': 
[2,4,6],'date2': ['12/31/2017','11/25/2019','10/06/2015'],'val2':[1,3,5],'date3': 
['12/31/2027','11/25/2029','10/06/2025'],'val3':[7,9,11]})


pd.wide_to_long(df, stubnames=['date', 'val'], i='person_id', 
j='grp').sort_index(level=0)
```
#### Problem description

winde_to_long retruns zero output when I change the column name format. The same code works when I have a different column name format. Please refer this post in SO for more details on list of column names that you can try. Replace the sample column names given in code with the column names in screenshot

https://stackoverflow.com/questions/56807104/wide-to-long-returns-empty-output-python-dataframe?noredirect=1#comment100169905_56807104

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#### Expected Output

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

<details>

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

</details>
