Added a check when Function's prototype have the 'async' property

SDK fails to send requests when used with some control-flow libraries e.g. [fibrous](https://github.com/goodeggs/fibrous), [node-sync](https://github.com/0ctave/node-sync).
That libraries extends Function's prototype to add 'async' function. 
In SequentialExecutor, I think that is better to check listener's async flag is the own property.