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

go-nntp/server/server.go

Lines 362 to 364 in c90cf61

	for k, v := range article.Header {
	fmt.Fprintf(dw, "%s: %s\r\n", k, v[0])
	}

go-nntp/server/server.go

Lines 445 to 447 in c90cf61

	for k, v := range article.Header {
	fmt.Fprintf(dw, "%s: %s\r\n", k, v[0])
	}