Fix pointers to alias types

This commit fixes pointers to type aliases which are currently not
supported.

The new test cases will fail with:
  jsonapi: Can't unmarshal foo (string) to struct field `String`, which is a pointer to `StringType (string)`

One other method to fix this when you are able to modify your types is:

  type StringType = string

This is the new (since Go 1.9) way to declare type aliasses which treats
the types differently when using reflect.
See: https://github.com/golang/example/tree/master/gotypes#named-types

models_test.go

@@ -5,6 +5,8 @@ import (
 	"time"
 )
 
+type StringType string
+
 type BadModel struct {
 	ID int `jsonapi:"primary"`
 }
@@ -18,11 +20,12 @@ type ModelBadTypes struct {
 }
 
 type WithPointer struct {
-	ID       *uint64  `jsonapi:"primary,with-pointers"`
-	Name     *string  `jsonapi:"attr,name"`
-	IsActive *bool    `jsonapi:"attr,is-active"`
-	IntVal   *int     `jsonapi:"attr,int-val"`
-	FloatVal *float32 `jsonapi:"attr,float-val"`
+	ID        *uint64     `jsonapi:"primary,with-pointers"`
+	Name      *string     `jsonapi:"attr,name"`
+	IsActive  *bool       `jsonapi:"attr,is-active"`
+	IntVal    *int        `jsonapi:"attr,int-val"`
+	FloatVal  *float32    `jsonapi:"attr,float-val"`
+	StringVal *StringType `jsonapi:"attr,string-val"`
 }
 
 type Timestamp struct {

request.go

@@ -580,7 +580,7 @@ func handlePointer(
 			reflect.ValueOf(attribute), fieldType, structField)
 	}
 
-	if t != concreteVal.Type() {
+	if !t.ConvertibleTo(concreteVal.Type()) {
 		return reflect.Value{}, newErrUnsupportedPtrType(
 			reflect.ValueOf(attribute), fieldType, structField)
 	}

request_test.go

@@ -45,6 +45,32 @@ func TestUnmarshall_attrStringSlice(t *testing.T) {
 	}
 }
 
+func TestUnmarshalToStructWithPointerToNamedType(t *testing.T) {
+	out := new(WithPointer)
+	in := map[string]interface{}{
+		"string-val": "foo",
+	}
+	if err := UnmarshalPayload(sampleWithPointerPayload(in), out); err != nil {
+		t.Fatal(err)
+	}
+	if *out.StringVal != "foo" {
+		t.Fatalf("Error unmarshalling to string alias ptr")
+	}
+}
+
+func TestUnmarshalToStructWithPointerToNamedTypeNull(t *testing.T) {
+	out := new(WithPointer)
+	in := map[string]interface{}{
+		"string-val": nil,
+	}
+	if err := UnmarshalPayload(sampleWithPointerPayload(in), out); err != nil {
+		t.Fatal(err)
+	}
+	if out.StringVal != nil {
+		t.Fatalf("Error unmarshalling to string alias ptr")
+	}
+}
+
 func TestUnmarshalToStructWithPointerAttr(t *testing.T) {
 	out := new(WithPointer)
 	in := map[string]interface{}{
@@ -68,6 +94,9 @@ func TestUnmarshalToStructWithPointerAttr(t *testing.T) {
 	if *out.FloatVal != 1.1 {
 		t.Fatalf("Error unmarshalling to float ptr")
 	}
+	if out.StringVal != nil {
+		t.Fatalf("Error unmarshalling to string alias ptr")
+	}
 }
 
 func TestUnmarshalPayload_ptrsAllNil(t *testing.T) {