Fix for edgecase with magic attributes

[pnomolos]

If you had a dragonfly accessor such as photo (with the associated photo_uid column)
but did not have a photo_size column, the defining a method such as thumbnail_photo_size
would include :size in the magic_attributes accessor for that model - even though one didn't
actually exist.

Ex:

class User
  dragonfly_accessor :photo

  def thumbnail_photo_size
    # The inclusion of this method causes Dragonfly to think this class has a photo_size column
  end
end