getters should use default values even if model was not properly initialized

[JelF]

steps to reproduce:

  Foo = Class.new
  Foo.include(Tainbox)
  dumped = Foo.new.to_yaml # => "--- !ruby/object:Foo {}\n"
  Foo.send(:attribute, :bar, default: :baz)

expected result

  YAML.load(dumped).bar # => :baz

actual result

  YAML.load(dumped).bar # => nil

possible workaround

def bar
  attribute_provided?(:bar) ? super : tainbox_set_default_bar
end