Correct cookie expiration date format

[adammeyer]

Cloudflare recently implemented cookie expiration date format validation and is removing expiration dates from cookies when they pass through if not in the correct format.

This corrects the date format to match the spec.

[akrabat]

Updated tests for the corrected date format.

[coveralls]

coverage: 99.893%. remained the same
when pulling 9f118ef on adammeyer:main
into 3b2fb2a on slimphp:main.

[odan]

Looks good to me.