fix calling interface on struct values that are private

diff_struct.go

@@ -54,6 +54,11 @@ func (d *Differ) diffStruct(path []string, a, b reflect.Value) error {
 			continue
 		}
 
+		// skip private fields
+		if !a.CanInterface() {
+			continue
+		}
+
 		err := d.diff(fpath, af, bf, a.Interface())
 		if err != nil {
 			return err

diff_test.go

@@ -6,6 +6,7 @@ package diff
 
 import (
 	"reflect"
+	"sync"
 	"testing"
 	"time"
 
@@ -51,6 +52,11 @@ type customTagStruct struct {
 	Bar int    `json:"bar"`
 }
 
+type privateValueStruct struct {
+	Public  string
+	Private *sync.RWMutex
+}
+
 type CustomStringType string
 type CustomIntType int
 type customTypeStruct struct {
@@ -352,6 +358,13 @@ func TestDiff(t *testing.T) {
 			},
 			nil,
 		},
+		{
+			"struct-with-private-value", privateValueStruct{Public: "one", Private: new(sync.RWMutex)}, privateValueStruct{Public: "two", Private: new(sync.RWMutex)},
+			Changelog{
+				Change{Type: UPDATE, Path: []string{"Public"}, From: "one", To: "two"},
+			},
+			nil,
+		},
 		{
 			"mismatched-values-struct-map", map[string]string{"test": "one"}, &tstruct{Identifiables: []tistruct{{"one", 1}}},
 			Changelog{},