Fix test failure due to incorrect tmpdir

[nanis]

This is the same issue as the one I addressed in d017587, but this particular instance was only exposed after 34e137e. See report on CPANTesters:

...
t\02_request\01_load.t .............................. ok
Error in tempfile() using template \XXXXXXXXXX: Could not create temp file \bosAojWwuw:
Permission denied at C:/opt/perl-5.22.0/site/lib/HTTP/Body/OctetStream.pm line 33.
# Looks like you planned 8 tests but ran 3.
# Looks like your test exited with 13 just after 3.
t\02_request\04_custom.t ............................ 
Dubious, test returned 13 (wstat 3328, 0xd00)
Failed 5/8 subtests 
t\02_request\04_forward.t ........................... ok
...

[bigpresh]

Bah, sorry - I missed this PR when I saw the issue report for this bug, and fixed it myself, with this exact fix - so I'll close the PR, but thanks!

[nanis]

Thank you very much for your work.