Added error checking for the result_data dictionary. Raises InvalidLTIConfigError if dictionary has more than one entry or if the entry does not have the key 'text' or 'url'. If url or text is set to None, the XML function inserts a self-closing tag (e.g. <url/>). If url or text set to empty string, the XML function puts both tags (e.g. <url></url>). I did not include url validation for the 'url' field because return URLs might be relative or absolute, depending on the application