def approve(user)
      friendship = find_any_friendship_with(user)
      return false if friendship.nil? || invited?(user)
      friendship.update_attribute(:pending, false)
    end
def approve(user)
      friendship = find_any_friendship_with(user)
      return false if friendship.nil? || invited?(user)
      friendship.update_attribute(:pending, false)
    end
approve a friendship invitation. If the operation succeeds, the method returns true, else false