Fix POP3 SASL authentication mechanism DIGEST-MD5. (closes #3862)

Fix POP3 SASL authentication mechanism DIGEST-MD5. (closes #3862)

sasl_client_step() returns SASL_OK after the fourth step: server auth
confirmation.  However, the protocol requires the client send one more
blank line to the server, to which the server then replies with "+OK".
See https://tools.ietf.org/html/rfc5034#section-6.

The code currently only sends a final response if sasl_client_step
returns data to send.  Change it to always send a final client message
after the SASL_OK.