Added missing import
Added test for creating ip address with port_id; also, some test cleanup
Added test for creating ip address with invalid port_id
Fixed typo in test method names
Added test for create_ip_address with none of network_id, device_id, nor port_id
Added test for create_ip_address with network_id without device_id
Added test for create_ip_address without network_id and with device_id; also, docstring cleanup
Added test for create_ip_address with version 4 specified
Added test for create_ip_address with version 6 specified