[GO][SERVER] Implement response code

[gmtstephane]

This PR implement the possibility to return custom response code.

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[wing328]

cc @antihax (2017/11) @grokify (2018/07) @kemokemo (2018/09) @bkabrda (2019/07)

[wing328]

Pushed 0d70d4a to update the samples. Let's see if that fixes the CI build failure.

[gmtstephane]

Thanks, looks like CI is fixed now :)

[wing328]

Looks good to start with. Let's merge with what you've added so far.