SASL challenge-response cycle does not process challenges

The SASL challenge-response cycle between a client peer and a server peer should look like this:

• client sends (possibly empty) response
• server evaluates response and sends challenge
• client evaluates challenge and returns response
  and so on until the cycle ends.

The client sends responses in SASL headers marked Type.RESPONSE.; the server sends challenges in SASL headers marked Type.CHALLENGE.

Due to a typo, all headers are marked as Type.RESPONSE, so that CHALLENGE messages were not being processed. The test case passes none the less!