GroupType.union(Type toMerge, boolean strict) does not honor strict parameter

This is the code as it currently stands in master:

```Java
@Override
protected Type union(Type toMerge, boolean strict) {
if (toMerge.isPrimitive()) {
throw new IncompatibleSchemaModificationException("can not merge primitive type " + toMerge + " into group type " + this);
}
return new GroupType(toMerge.getRepetition(), getName(), mergeFields(toMerge.asGroupType()));
}
```

Note the call to `mergeFields` omits the `strict` parameter. I believe the code should be:

```Java
@Override
protected Type union(Type toMerge, boolean strict) {
if (toMerge.isPrimitive()) {
throw new IncompatibleSchemaModificationException("can not merge primitive type " + toMerge + " into group type " + this);
}
return new GroupType(toMerge.getRepetition(), getName(), mergeFields(toMerge.asGroupType(), strict));
}
```

Note the call to `mergeFields` includes the `strict` parameter.

I would work on this myself, but I'm having considerable trouble working with the codebase (see e.g. http://stackoverflow.com/questions/31229445/build-failure-apache-parquet-mr-source-mvn-install-failure). Given the (assumed) simplicity of the fix, can a seasoned Parquet contributor take this up? Cheers.

**Reporter**: [Michael MacFadden](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=michael) / @mallman

_{**Note**: *This issue was originally created as [PARQUET-390](https://issues.apache.org/jira/browse/PARQUET-390). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Search the Parquet Java codebase for GroupType.union(Type toMerge, boolean strict), then inspect how mergeFields is called from that method. Confirm the strict value is propagated as described and run the relevant project tests to verify strict and non-strict schema merging behavior.

Written by the indexing model from the issue text.