provider/aws: Allow Internet Gateway IPv6 routes (#14484)

Fixes: #14006
Fixes: #14464

IPv6 wasn't supported for adding routes to the internet gateway.
Resulted in a message as follows:

```
Error creating route: MissingParameter: The request must contain the parameter destinationCidrBlock or destinationIpv6CidrBlock
```

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSRoute_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/15 11:50:43 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSRoute_ -timeout 120m
=== RUN   TestAccAWSRoute_basic
--- PASS: TestAccAWSRoute_basic (67.27s)
=== RUN   TestAccAWSRoute_ipv6Support
--- PASS: TestAccAWSRoute_ipv6Support (59.35s)
=== RUN   TestAccAWSRoute_ipv6ToInternetGateway
--- PASS: TestAccAWSRoute_ipv6ToInternetGateway (67.39s)
=== RUN   TestAccAWSRoute_changeCidr
--- PASS: TestAccAWSRoute_changeCidr (103.68s)
=== RUN   TestAccAWSRoute_noopdiff
--- PASS: TestAccAWSRoute_noopdiff (194.32s)
=== RUN   TestAccAWSRoute_doesNotCrashWithVPCEndpoint
--- PASS: TestAccAWSRoute_doesNotCrashWithVPCEndpoint (71.36s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	563.397s
```

builtin/providers/aws/resource_aws_route.go

@@ -133,10 +133,18 @@ func resourceAwsRouteCreate(d *schema.ResourceData, meta interface{}) error {
 	switch setTarget {
 	case "gateway_id":
 		createOpts = &ec2.CreateRouteInput{
-			RouteTableId:         aws.String(d.Get("route_table_id").(string)),
-			DestinationCidrBlock: aws.String(d.Get("destination_cidr_block").(string)),
-			GatewayId:            aws.String(d.Get("gateway_id").(string)),
+			RouteTableId: aws.String(d.Get("route_table_id").(string)),
+			GatewayId:    aws.String(d.Get("gateway_id").(string)),
 		}
+
+		if v, ok := d.GetOk("destination_cidr_block"); ok {
+			createOpts.DestinationCidrBlock = aws.String(v.(string))
+		}
+
+		if v, ok := d.GetOk("destination_ipv6_cidr_block"); ok {
+			createOpts.DestinationIpv6CidrBlock = aws.String(v.(string))
+		}
+
 	case "egress_only_gateway_id":
 		createOpts = &ec2.CreateRouteInput{
 			RouteTableId:                aws.String(d.Get("route_table_id").(string)),

builtin/providers/aws/resource_aws_route_test.go

@@ -86,6 +86,26 @@ func TestAccAWSRoute_ipv6Support(t *testing.T) {
 	})
 }
 
+func TestAccAWSRoute_ipv6ToInternetGateway(t *testing.T) {
+	var route ec2.Route
+
+	resource.Test(t, resource.TestCase{
+		PreCheck: func() {
+			testAccPreCheck(t)
+		},
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSRouteDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccAWSRouteConfigIpv6InternetGateway,
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSRouteExists("aws_route.igw", &route),
+				),
+			},
+		},
+	})
+}
+
 func TestAccAWSRoute_changeCidr(t *testing.T) {
 	var route ec2.Route
 	var routeTable ec2.RouteTable
@@ -288,6 +308,32 @@ resource "aws_route" "bar" {
 }
 `)
 
+var testAccAWSRouteConfigIpv6InternetGateway = fmt.Sprintf(`
+resource "aws_vpc" "foo" {
+  cidr_block = "10.1.0.0/16"
+  assign_generated_ipv6_cidr_block = true
+}
+
+resource "aws_egress_only_internet_gateway" "foo" {
+	vpc_id = "${aws_vpc.foo.id}"
+}
+
+resource "aws_internet_gateway" "foo" {
+  vpc_id = "${aws_vpc.foo.id}"
+}
+
+resource "aws_route_table" "external" {
+	vpc_id = "${aws_vpc.foo.id}"
+}
+
+resource "aws_route" "igw" {
+  route_table_id = "${aws_route_table.external.id}"
+  destination_ipv6_cidr_block = "::/0"
+  gateway_id = "${aws_internet_gateway.foo.id}"
+}
+
+`)
+
 var testAccAWSRouteConfigIpv6 = fmt.Sprintf(`
 resource "aws_vpc" "foo" {
   cidr_block = "10.1.0.0/16"