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.*}