webforms filter system for descriptions doesn't work with tinymce correctly

If tinymce is used to format the description text within a form formatting is not preserved by the webform filter call. This is because the call to filter_xss always uses the default filter list. This removes the

tags placed by tinymce. Instead a call should be made to check_markup to allow for variable html filter lists.

This change may justify the revision of my previous filter patch as a call to check_markup is no longer made in the description_filter function. If tinymce is not used the current filter process works correctly