fix problem revealed by tests

update() in a File asset would always create a storage location if one
was not yet saved. calling update() in a filter method would cause
another one to be created too, messing up the tests.

t/Asset/Asset.t

@@ -1017,6 +1017,7 @@ use HTML::Packer;
 my $asset   = WebGUI::Asset->getImportNode( $session )->addChild({
     className       => 'WebGUI::Asset::Snippet',
 });
+addToCleanup( $asset );
 my $unpacked = qq{<title>