Added support of unmarshalling and marshalling of pointers (#32)

During development of my own project I found some issues when trying to marshal php Nulls `N` into any Go struct fields. 

Was seeing uncaught panics such as `reflect: call of reflect.Value.Set on zero Value` if the value trying to be set on a struct field was Null.
Additionally if a value was being set on a struct field of type reflect.Ptr that was nil  a panic would occur when calling the .Set method on the struct field e.g. `reflect.Set: value of type string is not assignable to type *string`

I've added testing to showcase the now supported functionality:
* When serializing if a pointer value is Nil the MarshalNil function will be called
* When unserializing if the value passed in is a nil interface the struct field will be left unset (set as default value) rather then panic 
* If unmarshalling to a pointer struct field, the field will first be instantiated before attempting to set the value of the pointer

I used this Go playground to try base behaviour off of the `encoding/json` package: https://go.dev/play/p/5td0XqinlIP

Co-authored-by: danielbairstow97 <daniel@paypaplane.com>

consume.go

@@ -175,6 +175,11 @@ func setField(structFieldValue reflect.Value, value interface{}) error {
 	}
 
 	val := reflect.ValueOf(value)
+	if !val.IsValid() {
+		// structFieldValue will be set to default.
+		return nil
+	}
+
 	switch structFieldValue.Type().Kind() {
 	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
 		structFieldValue.SetInt(val.Int())
@@ -211,7 +216,10 @@ func setField(structFieldValue reflect.Value, value interface{}) error {
 		}
 
 		structFieldValue.Set(arrayOfObjects)
-
+	case reflect.Ptr:
+		// Instantiate structFieldValue.
+		structFieldValue.Set(reflect.New(structFieldValue.Type().Elem()))
+		return setField(structFieldValue.Elem(), value)
 	default:
 		structFieldValue.Set(val)
 	}

serialize.go

@@ -247,6 +247,9 @@ func Marshal(input interface{}, options *MarshalOptions) ([]byte, error) {
 		return MarshalStruct(input, options)
 
 	case reflect.Ptr:
+		if value.IsNil() {
+			return MarshalNil(), nil
+		}
 		return Marshal(value.Elem().Interface(), options)
 
 	default:

serialize_test.go

@@ -6,6 +6,10 @@ import (
 	"testing"
 )
 
+var (
+	heyStr = "hey"
+)
+
 type struct1 struct {
 	Foo    int
 	Bar    Struct2
@@ -33,6 +37,13 @@ type Struct3 struct {
 	StringArray []string
 }
 
+type Nillable struct {
+	Foo    string
+	Bar    Struct2
+	FooPtr *string
+	BarPtr *Struct2
+}
+
 type marshalTest struct {
 	input   interface{}
 	output  []byte
@@ -167,6 +178,18 @@ var marshalTests = map[string]marshalTest{
 		[]byte("O:8:\"stdClass\":3:{s:3:\"foo\";i:20;s:3:\"bar\";O:8:\"stdClass\":1:{s:3:\"qux\";d:7.89;}s:3:\"baz\";s:3:\"yay\";}"),
 		getStdClassOnly(),
 	},
+
+	// encode object with pointers
+	"Nillable{Foo string, Bar Struct2{Qux float64}, FooPtr *string, BarPtr *Struct2{Qux float64}": {
+		Nillable{"yay", Struct2{10}, &heyStr, &Struct2{}},
+		[]byte("O:8:\"Nillable\":4:{s:3:\"foo\";s:3:\"yay\";s:3:\"bar\";O:7:\"Struct2\":1:{s:3:\"qux\";d:10;}s:6:\"fooPtr\";s:3:\"hey\";s:6:\"barPtr\";O:7:\"Struct2\":1:{s:3:\"qux\";d:0;}}"),
+		nil,
+	},
+	"Nillable{Foo string, Bar Struct2{Qux float64}, FooPtr <nil>, BarPtr <nil>": {
+		Nillable{"", Struct2{}, nil, nil},
+		[]byte("O:8:\"Nillable\":4:{s:3:\"foo\";s:0:\"\";s:3:\"bar\";O:7:\"Struct2\":1:{s:3:\"qux\";d:0;}s:6:\"fooPtr\";N;s:6:\"barPtr\";N;}"),
+		nil,
+	},
 }
 
 func TestMarshal(t *testing.T) {

unserialize_test.go

@@ -537,6 +537,74 @@ func TestUnmarshalObjectWithTags(t *testing.T) {
 	}
 }
 
+func TestUnmarshalPointers(t *testing.T) {
+	data := "O:8:\"Nillable\":4:{s:3:\"foo\";s:3:\"yay\";s:3:\"bar\";O:7:\"Struct2\":1:{s:3:\"qux\";d:10;}s:6:\"fooPtr\";s:3:\"hey\";s:6:\"barPtr\";O:7:\"Struct2\":1:{s:3:\"qux\";d:0;}}"
+	target := &Nillable{
+		Foo: "yay",
+		Bar: Struct2{
+			Qux: 10,
+		},
+		FooPtr: &heyStr,
+		BarPtr: &Struct2{
+			Qux: 0,
+		},
+	}
+
+	var result Nillable
+	err := phpserialize.Unmarshal([]byte(data), &result)
+	expectErrorToNotHaveOccurred(t, err)
+
+	if result.Foo != target.Foo {
+		t.Errorf("Expected %v, got %v for Foo", target.Foo, result.Foo)
+	}
+
+	if result.Bar.Qux != target.Bar.Qux {
+		t.Errorf("Expected %v, got %v for Bar", target.Bar, result.Bar)
+	}
+
+	if result.FooPtr == nil || *result.FooPtr != *target.FooPtr {
+		t.Errorf("Expected %v, got %v for FooPtr", *target.FooPtr, *result.FooPtr)
+	}
+
+	if result.BarPtr == nil || result.BarPtr.Qux != result.BarPtr.Qux {
+		t.Errorf("Expected %v, got %v for BarPtr", target.BarPtr, result.BarPtr)
+	}
+
+}
+
+func TestUnmarshalPointersWithNull(t *testing.T) {
+	data := "O:8:\"Nillable\":4:{s:3:\"foo\";s:0:\"\";s:3:\"bar\";O:7:\"Struct2\":1:{s:3:\"qux\";d:0;}s:6:\"fooPtr\";N;s:6:\"barPtr\";N;}"
+
+	target := &Nillable{
+		Foo: "",
+		Bar: Struct2{
+			Qux: 0,
+		},
+		FooPtr: nil,
+		BarPtr: nil,
+	}
+
+	var result Nillable
+	err := phpserialize.Unmarshal([]byte(data), &result)
+	expectErrorToNotHaveOccurred(t, err)
+
+	if result.Foo != target.Foo {
+		t.Errorf("Expected %v, got %v for Foo", target.Foo, result.Foo)
+	}
+
+	if result.Bar.Qux != target.Bar.Qux {
+		t.Errorf("Expected %v, got %v for Bar", target.Bar, result.Bar)
+	}
+
+	if result.FooPtr != target.FooPtr {
+		t.Errorf("Expected %v, got %v for FooPtr", target.FooPtr, result.FooPtr)
+	}
+
+	if result.BarPtr != target.BarPtr {
+		t.Errorf("Expected %v, got %v for BarPtr", target.BarPtr, result.BarPtr)
+	}
+}
+
 func TestUnmarshalObjectIntoMap(t *testing.T) {
 	data := "O:7:\"struct1\":3:{s:3:\"foo\";i:10;s:3:\"bar\";O:7:\"Struct2\":1:{s:3:\"qux\";d:1.23;}s:3:\"baz\";s:3:\"yay\";}"
 	var result map[interface{}]interface{}