Fix mergeDefaults(config, defaults)

Hi!

Thanks for your awesome work.

I was testing your module with ghost driver running on a different port (5555).

So I tried to change to config object but the module didn't success to connect, so I jump to your code and I saw what's the problem was, and I fixed it for you :)

Have a nice day.

And don't forget to bump the version and publish on npm if you merged my commit.

Thanks for this awesome module guy!