text: Title: allow One hot encoder to work with nan
  Body: At the moment the one hot encoder does not support missing values. It will raise an error. We want to make this transformer more versatile.
  
  OneHotEncoder could have 3 options:
  
  - [ ] missing_values="raise" ==> current functionality
  - [ ] missing_values="encode" ==> treat na as an additional category
  - [ ] missing_values="ignore" ==> ignore, but only if encoding in k-1, otherwise they will be merged with the dropped category
  