Skip func type value in fields.

We skip those unprintable fields and an error field
instead of dropping the whole trace.

Fixes #642

logger_test.go

@@ -0,0 +1,42 @@
+package logrus
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestFieldValueError(t *testing.T) {
+	buf := &bytes.Buffer{}
+	l := &Logger{
+		Out:       buf,
+		Formatter: new(JSONFormatter),
+		Hooks:     make(LevelHooks),
+		Level:     DebugLevel,
+	}
+	l.WithField("func", func() {}).Info("test")
+	fmt.Println(buf.String())
+	var data map[string]interface{}
+	json.Unmarshal(buf.Bytes(), &data)
+	_, ok := data[FieldKeyLogrusError]
+	require.True(t, ok)
+}
+
+func TestNoFieldValueError(t *testing.T) {
+	buf := &bytes.Buffer{}
+	l := &Logger{
+		Out:       buf,
+		Formatter: new(JSONFormatter),
+		Hooks:     make(LevelHooks),
+		Level:     DebugLevel,
+	}
+	l.WithField("str", "str").Info("test")
+	fmt.Println(buf.String())
+	var data map[string]interface{}
+	json.Unmarshal(buf.Bytes(), &data)
+	_, ok := data[FieldKeyLogrusError]
+	require.False(t, ok)
+}