Display an error on charsets other than UTF-8

This prevents the proxy from sending invalid UTF-8 to the client.

main.go

@@ -453,8 +453,7 @@ func proxyGemini(req gemini.Request, external bool, root *url.URL,
 		return
 	}
 
-	// XXX: We could use the params I guess
-	m, _, err := mime.ParseMediaType(resp.Meta)
+	m, params, err := mime.ParseMediaType(resp.Meta)
 	if err != nil {
 		w.WriteHeader(http.StatusBadGateway)
 		w.Write([]byte(fmt.Sprintf("Gateway error: %d %s: %v",
@@ -468,6 +467,15 @@ func proxyGemini(req gemini.Request, external bool, root *url.URL,
 		return
 	}
 
+	if charset, ok := params["charset"]; ok {
+		charset = strings.ToLower(charset)
+		if charset != "utf-8" {
+			w.WriteHeader(http.StatusNotImplemented)
+			fmt.Fprintf(w, "Unsupported charset: %s", charset)
+			return
+		}
+	}
+
 	w.Header().Add("Content-Type", "text/html")
 	ctx := &GemtextContext{
 		CSS:      css,