Add RFC3339 timestamp (google#201)

@omarismail LGTM.

I'm curious what you think about perhaps documenting these `iso8601` and `rfc3339` in the `Readme.md`? How did you find that this tag option/value existed? How can we make this better for others vs having to search the library implementation?

Author: omarismail

Diff for: constants.go

@@ -9,8 +9,10 @@ const (
 	annotationRelation  = "relation"
 	annotationOmitEmpty = "omitempty"
 	annotationISO8601   = "iso8601"
+	annotationRFC3339   = "rfc3339"
 	annotationSeperator = ","
 
+	rfc3339TimeFormat = "2006-01-02T15:04:05Z07:00"
 	iso8601TimeFormat = "2006-01-02T15:04:05Z"
 
 	// MediaType is the identifier for the JSON API media type

Diff for: models_test.go

@@ -31,6 +31,12 @@ type Timestamp struct {
 	Next *time.Time `jsonapi:"attr,next,iso8601"`
 }
 
+type TimestampRFC3339 struct {
+	ID   int        `jsonapi:"primary,timestamps"`
+	Time time.Time  `jsonapi:"attr,timestamp,rfc3339"`
+	Next *time.Time `jsonapi:"attr,next,rfc3339"`
+}
+
 type Car struct {
 	ID    *string `jsonapi:"primary,cars"`
 	Make  *string `jsonapi:"attr,make,omitempty"`

Diff for: request.go

@@ -23,6 +23,9 @@ var (
 	// ErrInvalidISO8601 is returned when a struct has a time.Time type field and includes
 	// "iso8601" in the tag spec, but the JSON value was not an ISO8601 timestamp string.
 	ErrInvalidISO8601 = errors.New("Only strings can be parsed as dates, ISO8601 timestamps")
+	// ErrInvalidRFC3339 is returned when a struct has a time.Time type field and includes
+	// "rfc3339" in the tag spec, but the JSON value was not an RFC3339 timestamp string.
+	ErrInvalidRFC3339 = errors.New("Only strings can be parsed as dates, RFC3339 timestamps")
 	// ErrUnknownFieldNumberType is returned when the JSON value was a float
 	// (numeric) but the Struct field was a non numeric type (i.e. not int, uint,
 	// float, etc)
@@ -446,12 +449,15 @@ func handleStringSlice(attribute interface{}) (reflect.Value, error) {
 
 func handleTime(attribute interface{}, args []string, fieldValue reflect.Value) (reflect.Value, error) {
 	var isIso8601 bool
+	var isRFC3339 bool
 	v := reflect.ValueOf(attribute)
 
 	if len(args) > 2 {
 		for _, arg := range args[2:] {
 			if arg == annotationISO8601 {
 				isIso8601 = true
+			} else if arg == annotationRFC3339 {
+				isRFC3339 = true
 			}
 		}
 	}
@@ -476,6 +482,26 @@ func handleTime(attribute interface{}, args []string, fieldValue reflect.Value)
 		return reflect.ValueOf(t), nil
 	}
 
+	if isRFC3339 {
+		var tm string
+		if v.Kind() == reflect.String {
+			tm = v.Interface().(string)
+		} else {
+			return reflect.ValueOf(time.Now()), ErrInvalidRFC3339
+		}
+
+		t, err := time.Parse(time.RFC3339, tm)
+		if err != nil {
+			return reflect.ValueOf(time.Now()), ErrInvalidRFC3339
+		}
+
+		if fieldValue.Kind() == reflect.Ptr {
+			return reflect.ValueOf(&t), nil
+		}
+
+		return reflect.ValueOf(t), nil
+	}
+
 	var at int64
 
 	if v.Kind() == reflect.Float64 {

Diff for: request_test.go

@@ -413,6 +413,78 @@ func TestUnmarshalInvalidISO8601(t *testing.T) {
 	}
 }
 
+func TestUnmarshalParsesRFC3339(t *testing.T) {
+	payload := &OnePayload{
+		Data: &Node{
+			Type: "timestamps",
+			Attributes: map[string]interface{}{
+				"timestamp": "2020-03-16T23:09:59+00:00",
+			},
+		},
+	}
+
+	in := bytes.NewBuffer(nil)
+	json.NewEncoder(in).Encode(payload)
+
+	out := new(TimestampRFC3339)
+
+	if err := UnmarshalPayload(in, out); err != nil {
+		t.Fatal(err)
+	}
+
+	expected := time.Date(2020, 3, 16, 23, 9, 59, 0, time.UTC)
+
+	if !out.Time.Equal(expected) {
+		t.Fatal("Parsing the RFC3339 timestamp failed")
+	}
+}
+
+func TestUnmarshalParsesRFC3339TimePointer(t *testing.T) {
+	payload := &OnePayload{
+		Data: &Node{
+			Type: "timestamps",
+			Attributes: map[string]interface{}{
+				"next": "2020-03-16T23:09:59+00:00",
+			},
+		},
+	}
+
+	in := bytes.NewBuffer(nil)
+	json.NewEncoder(in).Encode(payload)
+
+	out := new(TimestampRFC3339)
+
+	if err := UnmarshalPayload(in, out); err != nil {
+		t.Fatal(err)
+	}
+
+	expected := time.Date(2020, 3, 16, 23, 9, 59, 0, time.UTC)
+
+	if !out.Next.Equal(expected) {
+		t.Fatal("Parsing the RFC3339 timestamp failed")
+	}
+}
+
+func TestUnmarshalInvalidRFC3339(t *testing.T) {
+	payload := &OnePayload{
+		Data: &Node{
+			Type: "timestamps",
+			Attributes: map[string]interface{}{
+				"timestamp": "17 Aug 16 08:027 MST",
+			},
+		},
+	}
+
+	in := bytes.NewBuffer(nil)
+	json.NewEncoder(in).Encode(payload)
+
+	out := new(TimestampRFC3339)
+
+	if err := UnmarshalPayload(in, out); err != ErrInvalidRFC3339 {
+		t.Fatalf("Expected ErrInvalidRFC3339, got %v", err)
+	}
+}
+
 func TestUnmarshalRelationshipsWithoutIncluded(t *testing.T) {
 	data, err := json.Marshal(samplePayloadWithoutIncluded())
 	if err != nil {