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.

Don't forget to bump the version and publish on npm if you merged my commit.

Thanks for this module guy!