PROTON-369: Ruby Message.clear clears out properties

Previously it was not clearing out the set of properties.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1508991 13f79535-47bb-0310-9956-ffa450edef68
PROTON-378: Add instructions property to Ruby Message class.

Also added Rspec tests.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1508992 13f79535-47bb-0310-9956-ffa450edef68