provider/aws: Fix ModifyInstanceAttribute on new instances

[grubernaut]

Previously ModifyInstanceAttribute permissions were required on creating a new instance with an unmodified source_dest_check attribute, as we forced the ModifyInstanceAttribute set on a new AWS instance.

This change only calls ModifyInstanceAttribute if source_dest_check was changed from default on a new instance, or if source_dest_check was modified.

$ make testacc TEST=./builtin/providers/aws TESTARGS="-run=TestAccAWSInstance_sourceDestCheck"
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/06/01 11:18:31 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSInstance_sourceDestCheck -timeout 120m
=== RUN   TestAccAWSInstance_sourceDestCheck
--- PASS: TestAccAWSInstance_sourceDestCheck (172.28s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    172.308s

Fixes: #9851

[stack72]

this is a great fix - nice work here :)

[ghost]

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