Do not ignore the `default` option for collection attributes

[Verseth]

I noticed that the default attribute option is ignored and replaced with -> { [] } for collection attributes.

I find it annoying, especially when someone wants the collection to be nil by default. Because the default option is disabled it is very hard to achieve.

I modified the behaviour so that collection attributes replace the default proc with -> { [] } only when default is nil.

This preserves the original behaviour of collection attributes having an empty array by default when no default option is passed. But the provided non-nil default is respected.

[kgiszczak]

Thank you, it looks good to me at first glance, but I'll have to review it more thoroughly. I don't remember right now, but this default might be required to not break some of the code paths.

[kgiszczak]

Hey so I did a quick test and indeed it crashes on XML parsing.

Here's an example how to reproduce:

class User < Shale::Mapper
  attribute :pets, Shale::Type::String, collection: true, default: -> { nil }

  xml do
    root 'user'
    map_element 'pet', to: :pets
  end
end

User.from_xml(<<~END)
<user>
  <pet>foo</pet>
  <pet>bar</pet>
  <pet>baz</pet>
</user>
END

It was just a quick test, but I feel like there might be other places in the code that assumes the default will be an array. You can take a stab at fixing it, but I feel it will be a pretty big lift. If you do, please create extensive test coverage so we have confidence in it.