Fixes 'Call-time pass-by-reference has been removed'

When trying to proxy a method which has declared references as arguments, the generated method call uses references as well. Because [call-time pass-by-reference](http://www.php.net/manual/en/language.references.pass.php) has been removed in PHP 5.4 a fatal error occurs.

```
Fatal error: Call-time pass-by-reference has been removed in /Volumes/Work/workspace/php-vcr/vendor/phpunit/phpunit-mock-objects/src/Framework/MockObject/Generator.php(301) : eval()'d code on line 52
```

Using `call_user_func_array()` avoids calling the method with references.