Inject the visitor rather than relying on other objects internals

This is ultimately messy no matter what, and increases the coupling to the
database backend, but we can at least contain it somewhat into an object
that's already coupled.

lib/arel/select_manager.rb

@@ -179,7 +179,7 @@ module Arel
     def where_sql engine = Table.engine
       return if @ctx.wheres.empty?
 
-      viz = Visitors::WhereSql.new engine.connection
+      viz = Visitors::WhereSql.new(engine.connection.visitor, engine.connection)
       Nodes::SqlLiteral.new viz.accept(@ctx, Collectors::SQLString.new).value
     end
 

lib/arel/visitors/where_sql.rb

@@ -1,12 +1,17 @@
 module Arel
   module Visitors
     class WhereSql < Arel::Visitors::ToSql
+      def initialize(inner_visitor, *args, &block)
+        @inner_visitor = inner_visitor
+        super(*args, &block)
+      end
+
       private
 
       def visit_Arel_Nodes_SelectCore o, collector
         collector << "WHERE "
         wheres = o.wheres.map do |where|
-          Nodes::SqlLiteral.new(@connection.visitor.accept(where, collector.class.new).value)
+          Nodes::SqlLiteral.new(@inner_visitor.accept(where, collector.class.new).value)
         end
 
         inject_join wheres, collector, ' AND '