Don't hard code xml URLs

The Shopify API supports JSON as well as XML,
these two method calls were causing clients to
receive XML responses, even when they had told
ActiveResource to use JSON.

lib/shopify_api.rb

@@ -190,7 +190,7 @@ module ShopifyAPI
   # the shop.
   class Shop < Base
     def self.current
-      find(:one, :from => "/admin/shop.xml")
+      find(:one, :from => "/admin/shop.#{format.extension}")
     end
     
     def metafields
@@ -423,7 +423,7 @@ module ShopifyAPI
       if args[0].is_a?(Symbol)
         super
       else
-        find(:one, :from => "/admin/assets.xml", :params => {:asset => {:key => args[0]}})
+        find(:one, :from => "/admin/assets.#{format.extension}", :params => {:asset => {:key => args[0]}})
       end
     end