Added fallback parameter to detectLanguage

Today I uncovered problem in my application, I relied that detectLanguage method always returns a language. But if the language can not be determined, then returns null.

It can be solved as follows
```php
$lang = $httpRequest->detectLanguage($languages);
if ($lang === NULL) {
    $lang = 'en';
}
```

Why not use a simpler
```php
$lang = $httpRequest->detectLanguage($languages, 'en');
```