HPCC-9607 IPropertyTree exception on using STD.System.Workunit.

Trim the value of xpath when request workunit because it contains illegal
space characters at the end of path string like
   'WorkUnits/W20130617-153029        '

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>
Merge pull request #4591 from AttilaVamos/HPCC-9607-fix

HPCC-9607 IPropertyTree exception on using STD.System.Workunit.

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>