MEDIUM: cache: Add "Origin" header to secondary cache key

MEDIUM: cache: Add "Origin" header to secondary cache key

This patch add a hash of the Origin header to the cache's secondary key.
This enables to manage store responses that have a "Vary: Origin" header
in the cache when vary is enabled.
This cannot be considered as a means to manage CORS requests though, it
only processes the Origin header and hashes the presented value without
any form of URI normalization.

This need was expressed by Philipp Hossner in GitHub issue #251.

Co-Authored-by: Philipp Hossner <philipp.hossner@posteo.de>