fix callback error with promise

[jeylost]

I didn't find promises support in the docs. But checking the code, I saw promise wrappers and tried to use them in my code. But there is a scenario when it will become an error. In the code, if statements use the callback function before the callback exists' check. I propose to create a promise wrapper as a default parameter and always return the result.

[coveralls]

Coverage increased (+5.2%) to 63.291% when pulling 80992db on jeylost:callback-promise-error into 953cc47 on ghdna:master.