SOLR-6780: Fixed a bug in how default/appends/invariants params were affecting the set of all keys found in the request parameters, resulting in some key=value param pairs being duplicated (merge r1642727)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene_solr_4_10@1643006 13f79535-47bb-0310-9956-ffa450edef68