unpickler: do not setattr() on tuples

Python 2.7 and newer does not allow doing setattr()
on instances of tuple().  We don't actually need to do it
since the namedtuple has already been completely decoded,
so returning early is desired.

This fixes the tests on Python 2.7.3.

Signed-off-by: David Aguilar <davvid@gmail.com>