(MCO-316) Allow username/password to be unspecified

In an environment where a user wants to set up authentication based
on client certificates, the current behavior of insisting on a
login/passcode being supplied falls down.

For rabbitmq especially this inhibits client certificate authentication
from working: https://www.rabbitmq.com/stomp.html#cta.ssl

    "Clients must not supply login and passcode headers."

Here we make the login/passcode optional and defaulting to '', which the stomp
gem takes to mean 'unspecified':

    https://github.com/stompgem/stomp/blob/v1.3.2/lib/connection/netio.rb#L354-L355

Note: for this to be effective you will need to use a stomp gem >=
1.3.2, as the behaviour of not passing login/passcode was fixed for
1.3.0 with this commit:

    https://github.com/stompgem/stomp/commit/7060e740a4e1ac3f3dd7a45a884bdbe5eb10ddc3
Merge pull request #190 from richardc/MCO-316_rabbitmq_certificates

(MCO-316) Allow username/password to be unspecified