Deep merging of nested exposures and presenter exposures (with :using option)

Suppose this case:

class GeneralInfo < Grape::Entity
  expose :gender, :age
end

class Student < Grape::Entity
  expose :info, using: GeneralInfo
  expose :info do
    expose :grade
  end
end

Student.represent({info: { gender: 'male', age: 25 }, grade: 5 }, serializable: true)
# => {:info=>{:grade=>5}}

But I assume info[:gender] and info[:age] to be exposed too. Also:

class Person < Grape::Entity
  expose :info do
    expose(:gender) { |obj| object[:info][:gender] }
    expose(:age) { |obj| object[:info][:age] }
  end
end

class Student < Person
  expose :info do
    expose :grade
  end
end

Student.represent({info: { gender: 'male', age: 25 }, grade: 5 }, serializable: true)
# => {:info=>{:gender=>"male", :age=>25, :grade=>5}}

Since we decided not to rewrite previously defined exposures in #151 then more consistent behavior would be make these cases act identical. It's good to think of presenter exposures (with :using option) as a nested exposures incapsulated in the separate classes.

Ones who want an old rewriting behavior should use conditional exposures (:if, :unless options).

Review issue #151 and the existing nested-exposure behavior before choosing an implementation approach. Reproduce both examples from this issue, then define the expected merged result for presenter exposures using :using and inherited exposures; done means both cases behave consistently without rewriting previously defined exposures.