text: Title: Porter stemmer returns a capital instead of lowercase 
  Body: This output is unexpected. The `In` returns the capitalize `In` from PorterStemmer's output.
  
  ```python
  >>> from nltk.stem import PorterStemmer
  >>> porter = PorterStemmer()
  >>> porter.stem('In')
  'In'
  ```
  
  More details on https://stackoverflow.com/q/60387288/610569