fix certificate caching, fixes #1033

[redcapital]

Looking at http://www.php-fig.org/psr/psr-6/ it seems getItem accepts only one parameter. The expiration must be set on the CacheItemInterface instance

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[redcapital]

I signed it!

[googlebot]

CLAs look good, thanks!

[mattwhisenhunt]

LGTM

[bshaffer]

This looks great, thank you!