-- additional tests to cover both variants of the set function
-- additional tests to cover hyper links on cells with non-string data
-- modified hyper link set function to handle cells with non-string data
-- fixed test behaviour for a cell with no value (value is set to the string form of the link. Behaviour copied from Excel 2010)
-- Resolves two test failures caused by a crash when setting the title to the existing value removing the sheet from the 'sheet_title_rel_id_map_'
-- added empty title check
