no projection in thrift union types.

We should not allow projecting a subset of the fields in a Thrift union as this will result in errors.
In a union there will be exactly one defined field.
Which means reading the data will fail if we project it out as Thrift expects exactly one (and not 0)

**Reporter**: [Julien Le Dem](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=julienledem) / @julienledem

_{**Note**: *This issue was originally created as [PARQUET-102](https://issues.apache.org/jira/browse/PARQUET-102). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}