Updates `aws_caller_identity` data source to actually include the correct attributes from the `GetCallerIdentity` API function.
```
$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSCallerIdentity_basic'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/27 09:26:13 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSCallerIdentity_basic -timeout 120m
=== RUN TestAccAWSCallerIdentity_basic
--- PASS: TestAccAWSCallerIdentity_basic (12.74s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 12.767s
```
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