`model.attribute?` accessor doesn't return true when attribute is present

[jonleighton]

Example code:

class Thing < PushType::Node
  field :name, :string
end

t = Thing.new
t.name? # => false, expected

t.name = ""
t.name? # => false, expected

t.name = "thing"
t.name? # => false, should be true

[aaronrussell]

Related to pr #30.