provider/aws: Allow Internet Gateway IPv6 routes

[stack72]

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

[grubernaut]

LGTM!

[ghost]

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