Update PTTL implementation to match Redis 2.8.4

Similar to 11f6941, update the implementation of `pttl` to match the
behavior of Redis 2.8.4. In the process, add more specs.

Change-Id: Ic40f83ec176504dc3d47df02b8ef9142586f2a1b
Reviewed-on: https://gerrit.causes.com/33213
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>