HPCC-10882 Allow cast real to string<n> if string preserves whole value

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5445 from ghalliday/issue10882

HPCC-10882 Allow cast real to string<n> if string preserves whole value

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>