PageCache storing headers in lower format

[samdark]

Discussed in https://github.com/yiisoft/yii2/discussions/19186

^{Originally posted by **nadar** January 23, 2022} We are using PageCache to cache a REST response, it seems that headers are always returned as lowercase chars afterwards, but the original input is case sensitive. https://github.com/yiisoft/yii2/blob/master/framework/filters/PageCache.php#L281 Whats the reason for this behavior? The default active data provider headers are f.e `X-Pagination-Page-Count` but when cached it returns `x-pagination-total-count` afterwards.

[nadar]

Thanks @bizley, very nice!