Fix bug in passing tuples to string.Template. All other values (with working

Fix bug in passing tuples to string.Template. All other values (with working
str() or repr()) would work, just not multi-value tuples. Probably not a
backport candidate, since it changes the behaviour of passing a
single-element tuple:

>>> string.Template("$foo").substitute(dict(foo=(1,)))

'(1,)'

versus

'1'