HPCC-8898 #EXPORT not handling decimal types correctly

The code for outputting basic type when exporting records to XML is weird.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4043 from richardkchapman/export-decimal

HPCC-8898 #EXPORT not handling decimal types correctly

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>