pnfsmanager: Fix tag handling in upload path creation

The map builder does not allow the same element to be added twice, which leads
to failures if WriteToken exists in both the target directory and the request.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: http://rb.dcache.org/r/6840/