fixed can't raise invalid expire time when set ex param is 0

[antsbean]

redis-cli
set py python ex 0 will raise invalid expire time in set
but
redis-py call set function
set('py', "python", ex=0) can't raise invalid expire time in set, lead to persist the key

I think this redis-py bug.

[NoneGG]

That‘s true, the problem can be reproduced.

[andymccurdy]

good catch. thanks!