SOLR-6182: correctly cast managedData as a List<Object> when loading stored RestManager data; solution verified with manual testing only as the unit tests use in-memory storage so will need to re-work the backing store to test this behavior in the unit test; backport to 4.9 branch

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene_solr_4_9@1604083 13f79535-47bb-0310-9956-ffa450edef68