Fixes to_json and to_xml for ActiveResource

Signed-off-by: José Valim <jose.valim@gmail.com>

activeresource/lib/active_resource/base.rb

@@ -552,19 +552,20 @@ def headers
       end
 
       def element_name
-        model_name.element
+        @element_name ||= model_name.element
       end
 
-      def element_name=(value)
-        model_name.element = value
+      def element_name=(element_name)
+        @element_name = element_name
+        @collection_name ||= model_name.collection.sub(/[^\/]*$/, @element_name.pluralize)
       end
 
       def collection_name
-        model_name.collection
+        @collection_name ||= model_name.collection
       end
 
-      def collection_name=(value)
-        model_name.collection = value
+      def collection_name=(collection_name)
+        @collection_name = collection_name
       end
 
       attr_accessor_with_default(:primary_key, 'id') #:nodoc:
@@ -1306,6 +1307,14 @@ def respond_to?(method, include_priv = false)
       end
     end
 
+    def to_json(options={})
+      super({ :root => self.class.element_name }.merge(options))
+    end
+
+    def to_xml(options={})
+      super({ :root => self.class.element_name }.merge(options))
+    end
+
     protected
       def connection(refresh = false)
         self.class.connection(refresh)