IV is not used to decrypt key

[sumbricht]

defaultKeyEncryptor.js uses the initialization vector for encryption, but not for decryption, which leads to strange errors downstream when using an EBICS client initialized with a non-empty IV.

I have fixed this locally and got it to work by just adding the IV there. Please see and merge my related PR

[leMaik]

Whoops… This was an oversight in #96, the PR looks fine. 👍