Calling ->district() on a CleverSchool object results in a CleverObject object, not a CleverDistrict object.

This code:

    $school = CleverSchool::retrieve('ABC123');
    $district = $school->district();
    var_dump($district);

results in:

    object(CleverObject)#6 (3) {
      ["_auth":protected]=>
      array(1) {
        ["apiKey"]=>
        string(32) "ABC123"
      }
      ["_values":protected]=>
      array(3) {
        ["id"]=>
        string(24) "123123123123123123"
        ["name"]=>
        string(21) "#DEMO Sandbox" 
        ["mdr_number"]=>
        string(0) ""
      }
      ["logger":protected]=>
      object(Psr\Log\NullLogger)#7 (0) {   
      }
    }

So, a CleverObject instead of the expected CleverDistrict.  

I'm not sure if this pull request is the best solution to the problem, but it seems to have solved the problem for me.