def unblock(user)
      friendship = find_any_friendship_with(user)
      return false if friendship.nil? || !friendship.can_unblock?(self)
      friendship.update_attribute(:blocker, nil)
    end
def unblock(user)
      friendship = find_any_friendship_with(user)
      return false if friendship.nil? || !friendship.can_unblock?(self)
      friendship.update_attribute(:blocker, nil)
    end
unblocks a friendship