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.*}

Durchsuche die Parquet-Java-Codebasis nach GroupType.union(Type toMerge, boolean strict) und untersuche anschließend, wie mergeFields von dieser Methode aufgerufen wird. Bestätige, dass der Wert von strict wie beschrieben weitergereicht wird, und führe die relevanten Projekttests aus, um das Verhalten der Schema-Zusammenführung im strict- und nicht-strict-Modus zu überprüfen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.