persisted? should be false after a document has been destroyed.

See https://github.com/rails/rails/blob/master/activemodel/CHANGELOG#L72

lib/couchrest/model/base.rb

@@ -82,7 +82,7 @@ module CouchRest
       end
 
       def persisted?
-        !new?
+        !new? && !destroyed?
       end
 
       def to_key

lib/couchrest/model/casted_model.rb

@@ -44,7 +44,7 @@ module CouchRest::Model
     alias :new_record? :new?
 
     def persisted?
-      !new?
+      !new? && !destroyed?
     end
 
     # The to_param method is needed for rails to generate resourceful routes.