SOLR-4426: NRTCachingDirectoryFactory does not initialize maxCachedMB and maxMergeSizeMB if <directoryFactory> is not present in solrconfig.xml

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1444786 13f79535-47bb-0310-9956-ffa450edef68