Bug: JSON serialization ignores fields from child classes (Java 8 Lambda runtime works, Java 11 Lambda runtime fails)

I noticed this issue when updating my lambda function from Java 8 to Java 11, and it seems to be related to the code in this repository.

The problematic situation occurs when a lambda function declares that it returns `TestParent`, but actually returns an instance of `TestChild`:

```java
public class TestParent {

public String parentField = "parentField";
}

public class TestChild extends TestParent {

public String childField = "childField";
}
```

To reproduce the issue, please run the attached test case (maven project): [aws-lambda-json-child-field-bug.zip](https://github.com/aws/aws-lambda-java-libs/files/9210528/aws-lambda-json-child-field-bug.zip)

It should fail with an error like below:
```
org.junit.ComparisonFailure:
Expected :{"parentField":"parentField","childField":"childField"}
Actual :{"parentField":"parentField"}
```

You can see that the fields from `TestChild` are missing. This is the issue, and this seems to be new -- in the Java 8 lambda runtime this worked ok.

Thanks for any help!