@only_fields and @except_fields memoization is wrong

[marshall-lee]

only_fields and except_fields methods internally memoizing @only_fields and @except_fields instance variables respectively. It's wrong because they're calculated against passed options parameter which can differ between separate serializable_hash calls.

I'd open a PR later.

[marshall-lee]

Failing example:

context 'with projections passed in options' do
  it 'allows to pass different :only and :except params using the same instance' do
    fresh_class.expose :a, :b, :c
    presenter = fresh_class.new({ a: 1, b: 2, c: 3})
    expect(presenter.serializable_hash(only: [:a, :b])).to eq(a: 1, b: 2)
    expect(presenter.serializable_hash(only: [:b, :c])).to eq(b: 2, c: 3)
  end
end