Fix type narrowing on case-when

[soutaro]

Add support for the case that more specific class is given to when clause.

# @type var object: Numeric | String
object = ""

case object
when Integer
  object + 1    # Make object be Integer here
else
  object        # object is Numeric | String because the when clause above cannot delete the Numeric case
end

[myronmarston]

Thanks for fixing this promptly!