Always set error on response in token create (#109) (#92)

If there was a network error, http response will be nil
but error needs to be propogated out.
diff --git a/openstack/identity/v3/tokens/requests.go b/openstack/identity/v3/tokens/requests.go
index b65de02..ba4363b 100644
--- a/openstack/identity/v3/tokens/requests.go
+++ b/openstack/identity/v3/tokens/requests.go
@@ -158,8 +158,8 @@
 	resp, err := c.Post(tokenURL(c), b, &r.Body, &gophercloud.RequestOpts{
 		MoreHeaders: map[string]string{"X-Auth-Token": ""},
 	})
+	r.Err = err
 	if resp != nil {
-		r.Err = err
 		r.Header = resp.Header
 	}
 	return