Make check of BooleanClause.Occur[] in MultiFieldQueryParser.parse less stubborn [LUCENE-2518]

Update the check in:

public static Query parse(Version matchVersion, String query, String[] fields,
BooleanClause.Occur[] flags, Analyzer analyzer) throws ParseException {
if (fields.length != flags.length)
throw new IllegalArgumentException("fields.length != flags.length");

To be:
if (fields.length > flags.length)

So the consumer can use one Occur array and apply fields selectively. The only danger here is with hitting a non-existent cell in flags, and this check will provide this just as well without limiting usability for such cases.

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Migrated from [LUCENE-2518](https://issues.apache.org/jira/browse/LUCENE-2518) by Itamar Syn-Hershko, updated Nov 30 2013

Start at MultiFieldQueryParser.parse(Version, String, String[], BooleanClause.Occur[], Analyzer), where the current fields.length != flags.length check is shown. Verify behavior for an Occur array shorter than the fields array and confirm that an array longer than the fields array remains acceptable; done means shorter arrays no longer fail this length check while missing flag access remains protected.

Written by the indexing model from the issue text.