Previously, the `stage_key` were not being set back to state in the Read
func. Changing this means the tests now run as follows:
```
make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSAPIGatewayApiKey_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestAccAWSAPIGatewayApiKey_ -timeout 120m
=== RUN TestAccAWSAPIGatewayApiKey_importBasic
--- PASS: TestAccAWSAPIGatewayApiKey_importBasic (42.42s)
=== RUN TestAccAWSAPIGatewayApiKey_basic
--- PASS: TestAccAWSAPIGatewayApiKey_basic (42.11s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 84.549s
```
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