client: fix handling of GRPC_GO_REQUIRE_HANDSHAKE=on to not interpret as "hybrid" (#2603)

6cc789b made `envconfig.RequireHandshakeOn` the default when unspecified by environment variable, but missed a fallthrough leading to `GRPC_GO_REQUIRE_HANDSHAKE=on` specifying `envconfig.RequireHandshakeHybrid`.  This change adds the missing fallthrough.

Author: dntj

Diff for: internal/envconfig/envconfig.go

@@ -59,6 +59,7 @@ var (
 func init() {
 	switch strings.ToLower(os.Getenv(requireHandshakeStr)) {
 	case "on":
+		fallthrough
 	default:
 		RequireHandshake = RequireHandshakeOn
 	case "off":