Fix FirefoxProfile::encode() so FireFoxProfile works for Windows Users as well

Some minor changes but this seems to have solved the issue that was raised in: https://github.com/facebook/php-webdriver/issues/165#.

There are some further possible improvements that can be made to encode() after extensive debugging of this function but I'll leave those open for another time. 
What is the problem of using `user.js`? 
According to mozilla (https://developer.mozilla.org/en-US/docs/Mozilla/Preferences/A_brief_guide_to_Mozilla_preferences), 
```
In the profile directory are two user pref files: prefs.js and user.js.  prefs.js is automatically generated by the application and should not be edited manually, whereas user.js is an optional file the user can create to override preferences initialized by other preferences files.
```

It looks more correct to use `user.js`. Also, the official Java binding is also using `user.js`.
See https://github.com/SeleniumHQ/selenium/blob/master/java/client/src/org/openqa/selenium/firefox/FirefoxProfile.java