Suggested by PR PHPOffice#1321 from @rtek, which was marked stale in March 2020. If the xml for a cell does not specify a type, PhpSpreadsheet has been using string; however, Excel uses numeric. PhpSpreadsheet will be changed to do the same as Excel. In theory, this is a BC break; in practice, it is doubtful that anyone will be adversely affected. No changes in the existing unit tests were required. The new test from rtek does demonstrate the change in behavior; however, the new behavior is certainly what a user would expect to happen. Unlike the original PR, this one is restricted to Xlsx Reader; I do not believe that any logic change is needed for Writer. (N.B. - changing Writer to use class constants rather than literals is a good idea, but, with no other logic change in this version, it seems risky - I may do that as separate PR.)