Use correct default initialize values

Default value of `local_host` and `local_port` of `TCPSocket` is `nil` according to http://ruby-doc.org/stdlib-2.0/libdoc/socket/rdoc/TCPSocket.html

Without this change, I got the error below. So please merge this pull request :)

```
Errno::EAFNOSUPPORT:
       Address family not supported by protocol family - bind(2)
```