let setex properly handle long expire values

adjust the format string passed to redis_cmd_format_static() to
properly handle long int, in order to prevent integer overflows
resulting in negative expire times passed to redis.
regression test for: setex properly handles long expire values
Merge branch 'hotfix/setex_long_expires'

Incorporate @zined and @s0enke commits to properly handle long timeout
values and not overrun our buffer.

Addresses #385