deep_merge does not function as expected on a Ruby Hash including Hashie::Extensions::DeepMerge

[BobbyMcWho]

While working #508, I discovered that deep merge does not correctly handle the following scenario properly when ActiveSupport is not present:

require 'bundler/inline'

gemfile(true) do
  source 'https://rubygems.org'
  gem 'hashie'
  gem 'pry'
end

require 'hashie'

class DeepHash < Hash
  include Hashie::Extensions::DeepMerge
end

h1 = DeepHash.new(a: 'a')
h2 = {a: 'a', b: {c: 'initial'}}

h2_c_id = h2.dig(:b, :c).object_id

h1 = h1.deep_merge!(h2)

h1_c_id = h1.dig(:b, :c).object_id

puts "h1 b-subhash id: #{h1_c_id}"
puts "h2 b-subhash id: #{h2_c_id}"

h1[:b][:c] = 'changed'

puts "h2[:b][:c] is #{h2.dig(:b, :c)}"

Fetching gem metadata from https://rubygems.org/............
Resolving dependencies...
Using bundler 1.17.2
Using coderay 1.1.2
Using hashie 4.0.0 (was 4.0.1)
Using method_source 0.9.2
Using pry 0.12.2
h1 b-subhash id: 70134510671180
h2 b-subhash id: 70134510671180
h2[:b][:c] is changed

which makes this test fail. This was hidden behavior due to the fact that we included active_support in our test suite and that was defining deep_merge on the hash instead. I added a fix as part of #508 since moving the activesupport specific tests into an integration test caused this test to fail (since I no longer loaded active_support).