constructor m_client(MAX_SOCK_NUM) is not portable (fails to compile with UIPEthernet)

the use of constructor EthernetClient(MAX_SOCK_NUM) breaks the compile when being used with UIPEthernet. Using the no-arg constructor has the same effect with Ethernet-library as it initializes _sock to MAX_SOCK_NUM anyway (https://github.com/arduino/Arduino/blob/master/libraries/Ethernet/EthernetClient.cpp#L17)