HPCC-12147 %a and %A options on std.date.tostring not returning
expected values

Fix date handling bug.

Add date2str test to regression suite.

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>
Merge pull request #6441 from AttilaVamos/HPCC-12147-fix

HPCC-12147 %a and %A options on std.date.tostring not returning

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>