Minor cleanup with tests

Added converter on saving in db user

spring-data-mongodb/src/main/java/org/baeldung/converter/UserWriterConverter.java

@@ -0,0 +1,25 @@
+package org.baeldung.converter;
+
+import org.springframework.stereotype.Component;
+import org.baeldung.model.User;
+import org.springframework.core.convert.converter.Converter;
+
+import com.mongodb.BasicDBObject;
+import com.mongodb.DBObject;
+
+@Component
+public class UserWriterConverter implements Converter<User, DBObject> {
+    @Override
+    public DBObject convert(User user) {
+        DBObject dbObject = new BasicDBObject();
+        dbObject.put("name", user.getName());
+        dbObject.put("age", user.getAge());
+        if (user.getEmailAddress() != null) {
+            DBObject emailDbObject = new BasicDBObject();
+            emailDbObject.put("value", user.getEmailAddress().getValue());
+            dbObject.put("email", emailDbObject);
+        }
+        dbObject.removeField("_class");
+        return dbObject;
+    }
+}