Format user arguments before passing them to string.format()

Format user arguments before passing them to string.format()

assert_(not_)equal and assert_(not_)match needs to convert the user
supplied arguments to strings when the error message is created.

Thanks to Gustavo Niemeyer <gustavo@niemeyer.net> for suggestions
and Chong Tsong <tsongchong@gmail.com> for pointing this out too.

Signed-off-by: Michael Roth <mroth@nessie.de>