Steep doesn't check assignment to Instance Variables

[radiantshaw]

Assume the following RBS file:

class Foobar
  @baz: Integer

  def initialize: (Integer baz) -> void
end

and the following Ruby file:

class Foobar
  def initialize(baz)
    @baz = baz.to_s
  end
end

The above should give a warning about assigning type ::String to an instance variable of type ::Integer, but it doesn't.