It should not be possible to project away columns of a key in a map in parquet-thrift

That wouldn't make much sense when re-assembling the map, there could be unexpected key collisions

**Reporter**: [Alex Levenson](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=alexlevenson) / @isnotinvain
**Assignee**: [Alex Levenson](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=alexlevenson) / @isnotinvain

_{**Note**: *This issue was originally created as [PARQUET-275](https://issues.apache.org/jira/browse/PARQUET-275). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

The issue names parquet-thrift but no source files, tests, or entry points. Start by locating the map projection handling and its existing tests; done means projections cannot remove columns from a map key and the behavior is covered by a regression test.

Written by the indexing model from the issue text.