`sendAjaxPostRequest` demands unnecessary `amOnPage`

When I start a functional test with
```php
$I->sendAjaxPostRequest(...);
```
...I'm getting the error message:
> [LogicException] The page history is empty.

So I'm forced to add a `$I->amOnPage($wherever);` before it. However, since `sendAjaxPostRequest()` gets an url as argument, it doesn't make any difference whatever I put into `amOnPage()`. So I'm suggesting to remove this requirement.
