Add a getter function for the Response status text.

The setStatusCode() method of the Response class sets a status code and a matching status text.
This variable is not declared by the class, and has no getter.

Since the variable is not declared as protected, I can do
```php
$status = $response->getStatusCode() . ' ' . $response->statusText;
```
in drupal, but it would be nicer to just be able to do $response->getStatusText(), no?