Add TestLogCopiesValuers

Useful to test; and helps me validate
that suffixes are always copied.

log/log_test.go

@@ -194,6 +194,43 @@ func TestWithConcurrent(t *testing.T) {
 	}
 }
 
+func TestLogCopiesValuers(t *testing.T) {
+	t.Parallel()
+	var output []interface{}
+	logger := log.Logger(log.LoggerFunc(func(keyvals ...interface{}) error {
+		output = keyvals
+		return nil
+	}))
+
+	valuerCallCount := 0
+	counterValuer := log.Valuer(func() interface{} {
+		valuerCallCount++
+		return valuerCallCount
+	})
+	lc := log.WithPrefix(logger, "a", counterValuer)
+	lc = log.WithSuffix(lc, "z", counterValuer)
+
+	if err := lc.Log(); err != nil {
+		t.Fatal(err)
+	}
+	want := []interface{}{"a", 1, "z", 2}
+	for i := 0; i < 4; i++ {
+		if want, have := want[i], output[i]; want != have {
+			t.Errorf("want output[%d] == %#v, have %#v", i, want, have)
+		}
+	}
+
+	if err := lc.Log(); err != nil {
+		t.Fatal(err)
+	}
+	want = []interface{}{"a", 3, "z", 4}
+	for i := 0; i < 4; i++ {
+		if want, have := want[i], output[i]; want != have {
+			t.Errorf("want output[%d] == %#v, have %#v", i, want, have)
+		}
+	}
+}
+
 func BenchmarkDiscard(b *testing.B) {
 	logger := log.NewNopLogger()
 	b.ReportAllocs()