NoMethodError: undefined method `[]=' for #<Grape::Entity::Options:0x007fc6a6f1a7b0>

When you have an entity which exposes an ActiveRecord belongs_to without a :using option, then serializable_hash is called with a Grape::Entity::Options object.

But serializable_hash clones the object and then tries to set :except on it. (It sets default :except to an empty array see here.) This causes the exception:

NoMethodError: undefined method '[]=' for #<Grape::Entity::Options:0x007fc6a6f1a7b0>

Not too big of a problem, it was just a forgotten :using, but it was very confusing.
Also depending on whether it's an object, array or hash, serializable_hash is called with or without options.

I'm talking about the code in Grape::Entity::Exposure::Base#serializable_value.

Should the options even be passed to serializable_hash at all? Or should []= be implemented?
This did work up until this commit btw.

Example to reproduce:

class Author < ActiveRecord::Base
end

class Post < ActiveRecord::Base
  belongs_to :author
end

class PostEntity < Grape::Entity
  expose :author # notice missing :using
end

PostEntity.represent(Post.first).as_json # => NoMethodError

Start in lib/grape_entity/exposure/base.rb at Exposure::Base#serializable_value and reproduce the failure with the Post, Author, and PostEntity example in the issue. Compare the object, array, and hash serialization paths and the linked ActiveRecord serializable_hash behavior. Done means the example no longer raises NoMethodError and the corrected behavior is covered by a regression test.