go/zodb: Fix Open to detect lack of schema only by ":" instead of by "://"

An URI schema is required to have ":" after it, but - even if frequently
used in practice - not //. We will soon introduce "demo:" URI scheme
that comes without //, so fix Open to detect schema presence just by ":"
and not to fixup "demo:..." url to "file://demo:..." automatically.

go/zodb/storage.go

@@ -100,7 +100,7 @@ func RegisterDriver(scheme string, opener DriverOpener) {
 // Storage authors should register their storages with RegisterStorage.
 func Open(ctx context.Context, zurl string, opt *OpenOptions) (IStorage, error) {
 	// no scheme -> file://
-	if !strings.Contains(zurl, "://") {
+	if !strings.Contains(zurl, ":") {
 		zurl = "file://" + zurl
 	}
 
@@ -115,7 +115,7 @@ func Open(ctx context.Context, zurl string, opt *OpenOptions) (IStorage, error)
 
 	opener, ok := driverRegistry[u.Scheme]
 	if !ok {
-		return nil, fmt.Errorf("zodb: URL scheme \"%s://\" not supported", u.Scheme)
+		return nil, fmt.Errorf("zodb: URL scheme \"%s:\" not supported", u.Scheme)
 	}
 
 	drvWatchq := make(chan Event)