Returning articles from the server discard some headers

[mildred]

Here, if an article has more than one header with the same name, the subsequent headers are discarded. They shouldn't. Also headers are scrambled due to the map data structure, see #9

https://github.com/dustin/go-nntp/blob/c90cf61f8fabcb90ab2a56309fb0697b71314698/server/server.go#L362-L364

https://github.com/dustin/go-nntp/blob/c90cf61f8fabcb90ab2a56309fb0697b71314698/server/server.go#L445-L447