Stop using a cached unpacker for MessagePackEventStream.

Sharing a a single instance of MesspaPack::Unpacker between input plugins and
MessagePackEventStream instances is unsafe. A test case has been added
to prove how easy it is leak events with the copy plugin and multiple
output plugins.
Merge pull request #137 from abecciu/copy-plugin-event-leak-fix

Fix copy plugin event leak and fix undefined Test::Unit::AssertionFailedError bug in ruby trunk.