expose_nil delegates to object, ignores methods defined in Entities

expose_nil delegates to the underlying object and doesn't test whether the method is delegatable? This can lead to odd behaviour and errors.

You'd probably have to be doing inheritance with your Entities in order to encounter it in the wild, but as a contrived example:

class MyEntity < ::Grape::Entity
  expose :foo, expose_nil: false

  def foo
    'forty-foo'
  end
end

class FooableClass
  def initialize(foo = nil)
    @foo = foo
  end

  def foo
    @foo
  end
end

class NoFooFoYou
end

# In all 3 of the following, I'd argue that the expected result is {:foo=>'forty-foo'}

MyEntity.represent(FooableClass.new(42)).as_json
# => {:foo=>'forty-foo'}

MyEntity.represent(FooableClass.new).as_json
# => {}

MyEntity.represent(NoFooFoYou.new).as_json
NoMethodError: undefined method `foo' for #<NoFooFoYou:>

Would you agree this is an issue?

Start with lib/grape_entity/exposure.rb at the linked expose_nil delegation code, then inspect how delegatable? handles methods defined on Entity classes. Reproduce the three MyEntity examples from the issue and verify that inherited Entity methods are respected without calling missing methods on the represented object.