Update 'once' listeners to accept a block argument

One-time listeners can now accept an emitted callback to match the behavior of persistent listeners. This appears to be the intended behavior based on this [change](https://github.com/igrigorik/http-2/commit/cd108d723dfd6d6cf475f545b4220e5022d6299a#diff-890759efb7afe0816c5f7c678dd89ab6R36).