GS: Use generic http transport

During the development of #1524 I discovered that the Google Cloud
Storage backend did not yet use the HTTP transport, so things such as
bandwidth limiting did not work. This commit does the necessary magic to
make the GS library use our HTTP transport.

cmd/restic/global.go

@@ -567,7 +567,7 @@ func open(s string, gopts GlobalOptions, opts options.Options) (restic.Backend,
 	case "s3":
 		be, err = s3.Open(cfg.(s3.Config), rt)
 	case "gs":
-		be, err = gs.Open(cfg.(gs.Config))
+		be, err = gs.Open(cfg.(gs.Config), rt)
 	case "azure":
 		be, err = azure.Open(cfg.(azure.Config), rt)
 	case "swift":
@@ -628,7 +628,7 @@ func create(s string, opts options.Options) (restic.Backend, error) {
 	case "s3":
 		return s3.Create(cfg.(s3.Config), rt)
 	case "gs":
-		return gs.Create(cfg.(gs.Config))
+		return gs.Create(cfg.(gs.Config), rt)
 	case "azure":
 		return azure.Create(cfg.(azure.Config), rt)
 	case "swift":