Fix race condition in munin output.

The munin server used the same structure in two goroutines at once causing
possible data corruption. A copy of the structure is now used by the second
goroutine instead.

plugins/munin/munin_test.go

@@ -0,0 +1,29 @@
+package munin
+
+import (
+	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/diebietse/invertergui/mk2driver"
+)
+
+func TestServer(t *testing.T) {
+
+	mockMk2 := mk2driver.NewMk2Mock()
+	muninServer := NewMunin(mockMk2)
+
+	ts := httptest.NewServer(http.HandlerFunc(muninServer.ServeMuninHTTP))
+	defer ts.Close()
+
+	res, err := http.Get(ts.URL)
+	if err != nil {
+		log.Fatal(err)
+	}
+	_, err = ioutil.ReadAll(res.Body)
+	res.Body.Close()
+	if err != nil {
+		log.Fatal(err)
+	}
+}