Add an engine spec for premature client removal.

While reviewing faye-redis-sharded code, I noticed that clients when
added were added with a time of 0, and not immediately pinged (like in
faye-redis). This spec verifies that a gc run after creating a client
won't remove the client.
Merge pull request #326 from zwily/test-for-premature-new-client-removal

Add an engine spec for premature client removal.