ASYNC-41: Make mult drop values when there are no taps

http://dev.clojure.org/jira/browse/ASYNC-41

If you have a mult and you put a value on its source channel, but there are no taps at the moment, the mult will get stuck and not put any values on future tap channels. Here's an example: http://cljsfiddle.net/fiddle/rads.mult-bug