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 :)




It was not working with this code : 
<pre>
var webdriver = require('selenium-node-webdriver');
webdriver({
  server: { port: 5555 }
})
.then(function (driver) {
  console.log('Connected!');
});
</pre>




Have a nice day.




And don't forget to bump the version and publish on npm if you merged my commit.




Thanks for this module guy!