[CHEF-3878] Add partial_search into Chef Search API.

With this patch, both the Chef Full Search API and Partial Search API work on Chef 11.6.2 :

1) Chef::Search::Query.new.search(:node, "*:*")
2) Chef::Search::Query.new.partial_search(:node, "*:*", keys: { name: ['name'] })

This patch and the search and partial_search API are tested in my project using Chef 11.6.2.