storage: use domain split domain

Use storage.googleapis.com in place of the default domain, unless
the user has specified another endpoint.

Change-Id: Ia62701588123104844e85cf303579e3c32f8e59b
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/48030
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Wilcox <crwilcox@google.com>
Reviewed-by: Chris Broadfoot <cbro@google.com>
Reviewed-by: Frank Natividad <franknatividad@google.com>
diff --git a/storage/storage.go b/storage/storage.go
index f40b7ca..59a859b 100644
--- a/storage/storage.go
+++ b/storage/storage.go
@@ -116,7 +116,11 @@
 	if err != nil {
 		return nil, fmt.Errorf("storage client: %v", err)
 	}
-	if ep != "" {
+	if ep == "" {
+		// Override the default value for BasePath from the raw client.
+		// TODO: remove when the raw client uses this endpoint as its default (~end of 2020)
+		rawService.BasePath = "https://storage.googleapis.com/storage/v1/"
+	} else {
 		rawService.BasePath = ep
 	}