Make extract! accept objects with internal hash implementation

[bacchir]

In a recent project I've experienced an odd situation of having an object that implements some Hash behavior and, even though it has methods such as [] and fetch, jbuilder.extract! did not work with it. So I just created this little PR with some tests to make jbuilder more agnostic.

Here's an example (also used in the tests) of the object I'm trying to pass to jbuilder:

class PersonWithHash
  attr_reader :name, :collection

  def initialize(name, age)
    @collection = { age: age }
    @name = name
  end

  delegate :[], :fetch, to: :@collection
end

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[bacchir]

@robin850 I changed the implementation the way you suggested, however I changed respond_to?(:fetch) to respond_to?(:[]) to avoid problems when an object have a custom fetch method for other purposes.