FedEx production use a different Soap URL

@umpirsky FedEx production use a different Soap URL. I need to manually set the URL to get my production keys to work. Is there a better way to handle it?

```php
public function calculate(AddressInterface $origin, AddressInterface $destination, ShippableInterface $shippable)
{
    $request = new Request();
    $request->getSoapClient()->__setLocation('https://ws.fedex.com:443/web-services/rate');
    $result = $request->getGetRatesReply(...);
}
```