Fix bug where container could be returned as an Array and not a Set.

Author: gkellogg

lib/json/ld/compact.rb

@@ -212,6 +212,7 @@ def compact(element,
             end
 
             container = context.container(item_active_property)
+            require 'byebug'; byebug if container && !container.is_a?(Set)
             as_array = !@options[:compactArrays] || context.as_array?(item_active_property)
 
             value = case

lib/json/ld/context.rb

@@ -1038,7 +1038,7 @@ def find_definition(term)
     # @param [Term, #to_s] term in unexpanded form
     # @return [Array<'@index', '@language', '@index', '@set', '@type', '@id', '@graph'>]
     def container(term)
-      return [term] if term == '@list'
+      return Set[term] if term == '@list'
       term = find_definition(term)
       term ? term.container_mapping : Set.new
     end