ChildLogger does not define <<

[rhuang]

Similar to https://github.com/tilfin/ougai/issues/112, it seems the child logger doesn't implement <<.

I'm running into an issue where I turned on http_wire_trace in the AWS Ruby SDK, which logs the wire trace in the net gem via <<:

https://github.com/ruby/net-http/blob/master/lib/net/http.rb#L2475