I have a class that uses `Str::random()` which I would like to test.

But when I use `Str::shouldReceive('random')` in my test, I get a BadMethodCallException saying the method shouldReceive does not exist.

I also tried to mock the class directly and bind it to the IOC but it keeps executing the original class, generating a random string and not the return value I set on the mock.

```
    $stringHelper = Mockery::mock('Illuminate\Support\Str');
    $this->app->instance('Illuminate\Support\Str', $stringHelper);
    //$this->app->instance('Str', $stringHelper);

    $stringHelper->shouldReceive('random')->once()->andReturn('some password');
    //Str::shouldReceive('random')->once()->andReturn('some password');

```