Change Block Storage v1 Volume.CreatedAt Type

This commit changes the Volume.CreatedAt type from
gophercloud.JSONRFC3339Milli to gophercloud.JSONRFC3339MilliNoZ.
diff --git a/openstack/blockstorage/v1/volumes/testing/fixtures.go b/openstack/blockstorage/v1/volumes/testing/fixtures.go
index 421cbf4..c841676 100644
--- a/openstack/blockstorage/v1/volumes/testing/fixtures.go
+++ b/openstack/blockstorage/v1/volumes/testing/fixtures.go
@@ -66,7 +66,7 @@
 			                "device": "/"
 			            }
 			        ],
-			        "created_at": "2012-02-14T20:53:07Z"
+			        "created_at": "2012-02-14T20:53:07"
 			    }
 			}
       `)