Fix go vet error in bender_test.go

go vet ./... was complaining. This fixes the issue.
```shell
go vet ./...
bender_test.go:43: arg m for printf verb %s of wrong type: *bender.EndRequestEvent
bender_test.go:47: arg m for printf verb %s of wrong type: *bender.EndRequestEvent
exit status 1
```