Date header: use current date when we don't get one handed over

When the content generator does not supply us with a date header,
we need to create one ourselves and set it to the current date.

Fixes:
https://github.com/pagespeed/ngx_pagespeed/issues/604 (duplicate)
https://github.com/pagespeed/ngx_pagespeed/issues/577
Merge pull request #605 from pagespeed/oschaaf-date-header

Date header: use current date when we don't get one handed over