Issue description

One of the the messages exchanged during the checking of PLAIN mechanism seems incorrect according to the RFC.

Environment

• libzmq version (commit hash if unreleased): 4.3.0
• OS: macOS 10.14.4

Minimal test code / Steps to reproduce the issue

Use the test code at https://github.com/zeromq/czmq/blob/master/examples/security/woodhouse.c/

1. Start a packet capturer (e.g. WireShark) and listen on the local loopback port 9000.
2. Compile and run the example code.
3. Examine the packet trace and locate the first message sent by the client after the greeting.

What's the actual result? (include assertion message & call stack if applicable)

The client sends "\x04\x15\x05WELCOME\x05admin\x06secret"

What's the expected result?

The client should have sent "\x04\x15\x05HELLO\x05admin\x06secret", according to the specification in 24/ZMTP-PLAIN.