These commits were based on a misunderstanding of the OCPP-J 1.6
specification, which clearly states that the password is a byte sequence
and not a string:

Revert "More password fixes."

This reverts commit fb5d0fb.

Revert "Fix password decoding."

This reverts commit cf20205.

Revert "Recommended by 1.6 spec is a 20 byte (40 chars) key."

This reverts commit f7b92a3.

This commit breaks the API, because the behaviour of the method is
changed to return the last configuration instead of the default
configuration:

Revert "A single instace, otherwise a static get() method makes no sense."

This reverts commit 953f50b.

OCPP 2.0.1 requires a CALLERROR: SecurityError response from the CSMS
if the Charging Station sends a request before having been accepted.

Implement a mechanism to send this response, by throwing the newly
added SecurityErrorException in the message handler of a disallowed
request.

This fixes issue ChargeTimeEU#350.