Fix marshalling TextMarshaler and Stringer

Fixes #7 by not marshalling those cases and let json.Marshal do it's job
instead.

sheriff.go

@@ -1,6 +1,7 @@
 package sheriff
 
 import (
+	"encoding"
 	"encoding/json"
 	"fmt"
 	"reflect"
@@ -157,9 +158,11 @@ func marshalValue(options *Options, v reflect.Value) (interface{}, error) {
 	if marshaller, ok := val.(Marshaller); ok {
 		return marshaller.Marshal(options)
 	}
-	// TODO(mweibel): This is a hack for struct types which conform to json.Marshaler (such as time.Time).
-	//                This makes sure those struct types aren't marshalled by sheriff if they appear as a field.
-	if _, ok := val.(json.Marshaler); ok {
+	// types which are e.g. structs, slices or maps and implement one of the following interfaces should not be
+	// marshalled by sheriff because they'll be correctly marshalled by json.Marshal instead.
+	// Otherwise (e.g. net.IP) a byte slice may be output as a list of uints instead of as an IP string.
+	switch val.(type) {
+	case json.Marshaler, encoding.TextMarshaler, fmt.Stringer:
 		return val, nil
 	}
 	k := v.Kind()

sheriff_test.go

@@ -2,6 +2,7 @@ package sheriff
 
 import (
 	"encoding/json"
+	"net"
 	"testing"
 	"time"
 
@@ -498,3 +499,30 @@ func TestMarshal_InlineStruct(t *testing.T) {
 
 	assert.Equal(t, string(expected), string(actual))
 }
+
+type TestInet struct {
+	IPv4 net.IP `json:"ipv4"`
+	IPv6 net.IP `json:"ipv6"`
+}
+
+func TestMarshal_Inet(t *testing.T) {
+	v := TestInet{
+		IPv4: net.ParseIP("0.0.0.0").To4(),
+		IPv6: net.ParseIP("::").To16(),
+	}
+	o := &Options{}
+
+	actualMap, err := Marshal(o, v)
+	assert.NoError(t, err)
+
+	actual, err := json.Marshal(actualMap)
+	assert.NoError(t, err)
+
+	expected, err := json.Marshal(map[string]interface{}{
+		"ipv4": net.ParseIP("0.0.0.0").To4(),
+		"ipv6": net.ParseIP("::").To16(),
+	})
+	assert.NoError(t, err)
+
+	assert.Equal(t, string(expected), string(actual))
+}