Regenerate @_lookup_cache after each manipulation

Trying to call Resource#metadata from within
MyExtension#manipulate_resource_list generates an exception in
Proxies::ResourceInstanceMethods#get_source_file because @_lookup_cache
is empty. Moving the recalculation inside the loop means regenerating
the cache after each manipulation but allows extensions to examine the
page metadata when manipulating proxied resources.

middleman-core/lib/middleman-core/sitemap/store.rb

@@ -47,15 +47,17 @@ module Middleman
       # @return [void]
       def rebuild_resource_list!(reason=nil)
         @resources = @resource_list_manipulators.inject([]) do |result, (_, inst)|
-          inst.manipulate_resource_list(result)
-        end
+          newres = inst.manipulate_resource_list(result)
 
           # Reset lookup cache
           @_lookup_cache = { :path => {}, :destination_path => {} }
-        @resources.each do |resource|
+          newres.each do |resource|
             @_lookup_cache[:path][resource.path] = resource
             @_lookup_cache[:destination_path][resource.destination_path] = resource
           end
+
+          newres
+        end
       end
     
       # Find a resource given its original path