Fix obscure bug where the last outgoing message (and any capabilities therein) would not get released (until a new message was sent, replacing it as the last).

This took hours to track down, because it initially looked like "Release" messages weren't being honored in some cases (when they happened to be releasing a capability from the last message, and no subsequent messages were sent). Initial attempts to capture this in a unit test failed because the test of course used a subsequent call to detect if the capability had been released, which succeeded.