allow unencrypted alerts just before encrypted messages

[tomato42]

In some situations, the other side of the connection may send an unencrypted alert when we expect encrypted messages, that causes the decryption to fail instead of returning the alert to the calling application.

Fixes #344

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This change is 

[ep69]

I am not sure about the change header.type != ContentType.change_cipher_spec -> header.type == ContentType.application_data. Could it perhaps run into some of the other ContentType that was not mentioned, like handshake or heartbeat?

[tomato42]

1. all encrypted messages in TLS 1.3 have the external content type of application_data (and that's what we're checking there), so no, the change is correct
2. no, heartbeat is not supported in TLS 1.3