Metadata keys can have multiple values

Closes #6

http/handler.go

@@ -112,10 +112,10 @@ func (this *Handler) serveDocument (
 		recorder.Reset()
 		recorder.Head = res.Header().Clone()
 	}
-	if contentType, ok := document.Meta["content-type"]; ok {
+	if contentType := document.Meta.Get("content-type"); contentType != "" {
 		recorder.Header().Set("Content-Type", contentType)
 	}
-	if status, ok := document.Meta["status"]; ok {
+	if status := document.Meta.Get("status"); status != "" {
 		if status, err := strconv.Atoi(status); err == nil {
 			recorder.Status = status
 		}