Fix go compatibility for <= 1.11

Author: brandonc

response.go

@@ -406,7 +406,7 @@ func visitModelNode(model interface{}, included *map[string]*Node,
 					choiceValue := fieldValue
 
 					// must be a pointer type
-					if choiceValue.Type().Kind() != reflect.Pointer {
+					if choiceValue.Type().Kind() != reflect.Ptr {
 						er = ErrUnexpectedType
 						break
 					}
@@ -430,7 +430,7 @@ func visitModelNode(model interface{}, included *map[string]*Node,
 					for i := 0; i < fieldValue.Len(); i++ {
 						itemValue := fieldValue.Index(i)
 						// Once again, must be a pointer type
-						if itemValue.Type().Kind() != reflect.Pointer {
+						if itemValue.Type().Kind() != reflect.Ptr {
 							er = ErrUnexpectedType
 							break
 						}

response_test.go

@@ -3,7 +3,6 @@ package jsonapi
 import (
 	"bytes"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"reflect"
 	"sort"
@@ -150,7 +149,7 @@ func TestMarshalPayloadWithManyPolyrelationWithNils(t *testing.T) {
 	}
 
 	out := bytes.NewBuffer(nil)
-	if err := MarshalPayload(out, blog); !errors.Is(err, ErrUnexpectedNil) {
+	if err := MarshalPayload(out, blog); err != ErrUnexpectedNil {
 		t.Fatal("expected error but got none")
 	}
 }
@@ -169,7 +168,7 @@ func TestMarshalPayloadWithManyRelationWithNils(t *testing.T) {
 	}
 
 	out := bytes.NewBuffer(nil)
-	if err := MarshalPayload(out, blog); !errors.Is(err, ErrUnexpectedNil) {
+	if err := MarshalPayload(out, blog); err != ErrUnexpectedNil {
 		t.Fatal("expected error but got none")
 	}
 }