ProtoRecordConverter might wrongly cast a Message.Builder to Message

ProtoRecordConverter returns current record as follows:

```Java
public T getCurrentRecord() {
if (buildBefore) {
return (T) this.reusedBuilder.build();
} else {
return (T) this.reusedBuilder;
}
}
```

However this might fail if T is subclass of Message and buildBefore == false, since it's actually casting a Message.Builder instance to Message type.

**Reporter**: [Wu Xiang](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=wxiang7)
**Assignee**: [Reuben Kuhnert](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=sircodesalot) / @sircodesalotOfTheRound

_{**Note**: *This issue was originally created as [PARQUET-388](https://issues.apache.org/jira/browse/PARQUET-388). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start at ProtoRecordConverter#getCurrentRecord and inspect the reusedBuilder and buildBefore paths described in the issue. Reproduce the case where the current record type is a Message subclass and buildBefore is false, then verify that the returned value has the expected Message type without a cast failure.

Written by the indexing model from the issue text.