MockMemcacheClient.set: assert that expire is an integer

[snarfed]

the real client does this, so ideally the mock should too.

[snarfed]

@jparise type annotation sounds great! I didn't go that route originally because MockMemcacheClient doesn't currently have any, and it felt awkward to add just one in isolation, and I wasn't quite ready to tackle the whole class. I'm open to anything though.