SystemStackLevel exception if Hashie::Extensions::Coercion is included in parent class and coercion target

[maxlinc]

Problem:

I came across a situation where coercion blows up with a nasty and cryptic error:

  1) Hashie::Extensions::Coercion#coerce_key nesting coeces nested objects
     Failure/Error: Unable to find matching line from backtrace
     SystemStackError:
       stack level too deep
     # ./lib/hashie/extensions/coercion.rb:14

I found it while working on coercions for core types, but I verified the problem exists on master.

Test:

    context 'nesting' do
      class BaseCoercableHash < Hash
        include Hashie::Extensions::Coercion
        include Hashie::Extensions::MergeInitializer
      end

      class NestedCoercableHash < BaseCoercableHash
        include Hashie::Extensions::Coercion
        include Hashie::Extensions::MergeInitializer
        # coerce_key :foo, Integer
      end

      class RootCoercableHash < BaseCoercableHash
        include Hashie::Extensions::Coercion
        include Hashie::Extensions::MergeInitializer
        coerce_key :nested, NestedCoercableHash
      end

      subject { RootCoercableHash }
      let(:instance) { subject.new }

      it 'coeces nested objects' do
        subject.coerce_key :nested, NestedCoercableHash

        instance[:nested] = { foo: '123' }
        expect(instance[:nested]).to be_a(NestedCoercableHash)
        # expect(instance[:nested][:foo]).to be_an(Integer)
        # expect(instance[:nested][:foo]).to eq('123')
      end
    end

Workaround:

Remove include Hashie::Extensions::Coercion from the subclasses so it's only in BaseCoercableHash.

That's more DRY anyways, but I don't think it's illegal to include the same module twice... in fact I just tested and you can also create this error by including Hashie::Extensions::Coercion twice in the base class, while including Hashie::Extensions::MergeInitializer twice doesn't cause a problem.

Solution?:

Ideally this code should work, or at least raise a more useful specific error.

If that's not possible it could just be documented as a known issue.

[dblock]

This looks like a legit problem, would appreciate a PR with a fix, or even with just the test that reproduces the issue as above turned into a PR.