Make BloomFilteringPostingsFormat wrap Lucene40PostingsFormat by default [LUCENE-4394]

When loading BloomFilteringPostingsFormat from the SPI registry, it will have a null delegate by default. I think it would be more comfortable to make it wrap Lucene40PostingsFormat by default so that it can be used against lucene-core tests (ant -Dtests.postingsFormat=BloomFilter, currently fails with "java.lang.UnsupportedOperationException: Error - org.apache.lucene.codecs.bloom.BloomFilteringPostingsFormat has been constructed without a choice of PostingsFormat") or with Solr field types with minimal configuration (<fieldType ... postingsFormat="BloomFilter" />).

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Migrated from [LUCENE-4394](https://issues.apache.org/jira/browse/LUCENE-4394) by Adrien Grand (@jpountz), updated Sep 17 2012

Start by reading BloomFilteringPostingsFormat and reproduce the reported failure with `ant -Dtests.postingsFormat=BloomFilter`. Check how the SPI registry constructs the format and verify that the lucene-core tests and the stated Solr field-type configuration work without additional postings-format configuration.

Written by the indexing model from the issue text.