Boolean format in Plain Decoder

In the encoding.md document is written that the plain encoder for boolean uses [RLE/BitPacking](https://github.com/apache/parquet-format/blob/master/Encodings.md#plain-plain--0)

While in the cpp implementation seems is just using [simple bit decoding back to back.](https://github.com/apache/parquet-cpp/blob/master/src/parquet/encodings/plain-encoding.h#L151)

Which one is the right format ?

**Reporter**: [Fabrizio Milo](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=fmilo)

_{**Note**: *This issue was originally created as [PARQUET-574](https://issues.apache.org/jira/browse/PARQUET-574). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}