use keySet() instead stringPropertyNames because stringPropertyNames cant get the property that was put with put()

Author: katamotokosuke

src/main/java/org/embulk/output/SnowflakeOutputPlugin.java

@@ -186,7 +186,8 @@ protected BatchInsert newBatchInsert(PluginTask task, Optional<MergeConfig> merg
   @Override
   protected void logConnectionProperties(String url, Properties props) {
     Properties maskedProps = new Properties();
-    for (String key : props.stringPropertyNames()) {
+    for (Object keyObj : props.keySet()) {
+      String key = (String) keyObj;
       if (key.equals("password")) {
         maskedProps.setProperty(key, "***");
       } else if (key.equals("proxyPassword")) {