Password is destroyed when PowerAuthAuthentication is released by GC

[hvge]

If PowerAuthAuthentication object is being released by GC, then it also destroys assigned Password object. This is highly unwanted behavior, because the Password object instance might be referenced elsewhere in the application. That may lead to a random authentication failures as we encountered in #606

[hvge]

After a deeper analysis I think that this will be triggered only if application is keeping its own Password object and try to sign two HTTP requests with the same password in a sequence. The second attempt may randomly fail on a mystic exception PowerAuthErrorException: Session is no longer valid.