Read secret.Data since secret.StringData is write-only

As described in
https://github.com/kubernetes/kubernetes/blob/4b00d5e42a4bfdcecf061b018911cbdeeac1d9f3/pkg/api/v1/types.go#L4060
StringData is write-only and never output when reading.

builtin/providers/kubernetes/resource_kubernetes_secret.go

@@ -79,7 +79,7 @@ func resourceKubernetesSecretRead(d *schema.ResourceData, meta interface{}) erro
 		return err
 	}
 
-	d.Set("data", secret.StringData)
+	d.Set("data", secret.Data)
 
 	return nil
 }