The volume_Droplet test was failing from an incorrect fix pushed on Friday. Fixes the failing test.
```
$ make testacc TEST=./builtin/providers/digitalocean TESTARGS='-run=TestAccDigitalOceanVolume_Droplet'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/02/11 17:55:09 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/digitalocean -v -run=TestAccDigitalOceanVolume_Droplet -timeout 120m
=== RUN TestAccDigitalOceanVolume_Droplet
--- PASS: TestAccDigitalOceanVolume_Droplet (52.78s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/digitalocean 52.797s
```
