Charset was removed from Content-Type intentionally in latest slim 3.x version. See https://github.com/slimphp/Slim/issues/2629 for reasons.
Ensure that composer respects our PHP support commitment.
Therefore sha256 hash of encryption key is validated against one which is stored
on server on poll creation.
This one is transfered as X-Croodle-Proof-Key-Knowledge HTTP HEADER.
Prevents an attacker of transmitting data with wrong encryption key, which
would cause decryption errors for legit users.
therefore we have to duplicate it in store:
* encrypted for to serve for clients (encryptedExpirationDate)
* unencrypted for server to check if it's exceeded (serverExpirationDate)
serverExpirationDate should never be send to client
