Ensure stubbed properties set the proper status code.

By raising an exception (HTTPStatus derived class) the renderer will
generate an empty tag and attach it to the proper HTTP status.  Merely
returning the error code will cause the renderer to create a tag with
the textual contents being the HTTPStatus class.

lib/dav4rack/resource.rb

@@ -140,22 +140,23 @@ def parent_exists?
 
     # Return the creation time.
     def creation_date
-      NotImplemented
+      raise NotImplemented
     end
 
     # Return the time of last modification.
     def last_modified
-      NotImplemented
+      raise NotImplemented
     end
 
     # Set the time of last modification.
     def last_modified=(time)
-      NotImplemented
+      # Is this correct?
+      raise NotImplemented
     end
 
     # Return an Etag, an unique hash value for this resource.
     def etag
-      NotImplemented
+      raise NotImplemented
     end
 
     # Return the resource type. Generally only used to specify
@@ -166,12 +167,12 @@ def resource_type
 
     # Return the mime type of this resource.
     def content_type
-      NotImplemented
+      raise NotImplemented
     end
 
     # Return the size in bytes for this resource.
     def content_length
-      NotImplemented
+      raise NotImplemented
     end
 
     # HTTP GET request.