Add a valid proxy into pool

        You must call `add_proxy` method to add a proxy into pool instead of
        directly operate the `proxies` variable.

def add_proxy(self, proxy):
        """Add a valid proxy into pool

        You must call `add_proxy` method to add a proxy into pool instead of
        directly operate the `proxies` variable.
        """
        protocol = proxy.protocol
        addr = proxy.addr
        if addr in self.proxies:
            self.proxies[protocol][addr].last_checked = proxy.last_checked
        else:
            self.proxies[protocol][addr] = proxy
            self.addr_list[protocol].append(addr)