multi succeeds twice bug

Multi succeeds twice if request is performed before adding. For example, when defferable object has cache, it may get data without making remote request. In this case, callback body is called earlier than connection is added to @requests hash. 

em-http-request contains logic without this bug. See https://github.com/igrigorik/em-http-request/blob/master/lib/em-http/multi.rb