spec: strptime: improve '%s %z' test

We can test both the time and timezone at the same time, and also, it
makes sense to do the succinct test of checking back-to-back conversion
of strptime().strftime().

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
strptime: synchronize with MRI

It turns out that '%s' in C returns a local time, and strptime(3)
doesn't say that '%s' implies UTC. So it makes sense to keep the old
behavior for this case, and it's also safer.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Merge pull request #3 from felipec/strptime-improve

Strptime improvement