Support unix fd as method parameter in own services

Without this patch i cannot implement a service which contains a method that takes an unix fd as a parameter, e.g.

```ruby
dbus_method :NewConnection, "in device:o, in fd:h, in fd_properties:a{sv}" do |device, fd, fd_properties|
    puts "NewConnection"
end
```

This method is never called. Insted the following message appears in the dbus log:
```
Serial Port replied with an error: org.freedesktop.DBus.Error.NotSupported, Tried to send message with Unix file descriptorsto a client that doesn't support that.
```

This patch fixes the problem for me.

See [D-Bus documentation](http://dbus.freedesktop.org/doc/dbus-specification.html#auth-command-agree-unix-fd)
See [My example code](https://gist.github.com/chrta/8897809)