Make Model#dup copy the attributes too.

Otherwise, #dup-ing yourself and operating on a copy isn't enough
to ensure your own state remains untouched. For example,
AWS::Storage::Files#each will only work once without this change, since
even though the object operates on a copy of itself, its 'marker' attribute
winds up getting changed because @attributes is shared with the copy.

lib/fog/core/attributes.rb

@@ -121,6 +121,12 @@ module Fog
         @attributes ||= {}
       end
 
+      def dup
+        copy = super
+        copy.attributes = attributes.dup
+        copy
+      end
+
       def identity
         send(self.class.instance_variable_get('@identity'))
       end
@@ -165,6 +171,10 @@ module Fog
         end
       end
 
+      protected
+
+      attr_writer :attributes
+
       private
 
       def remap_attributes(attributes, mapping)