Can't json_encode WP_Error in WP4.0


WP 4.0:
JSON
```[10-Sep-2014 02:28:53 UTC] {"get_backup":{}}```

VAR:
```[10-Sep-2014 02:28:53 UTC] array (
  'get_backup' => 
  WP_Error::__set_state(array(
     'errors' => 
    array (
      'error-status' => 
      array (
        0 => 'Creating zip archive (<code>zip</code>)',
      ),
    ),
     'error_data' => 
    array (
    ),
  )),
)```

WP 3.9:
JSON
```[10-Sep-2014 02:30:01 UTC] {"get_backup":{"errors":{"error-status":["Creating zip (<code>zip<\/code>)"]},"error_data":[]}}```

VAR
```[10-Sep-2014 02:30:01 UTC] array (
  'get_backup' => 
  WP_Error::__set_state(array(
     'errors' => 
    array (
      'error-status' => 
      array (
        0 => 'Creating zip archive (<code>zip</code>)',
      ),
    ),
     'error_data' => 
    array (
    ),
  )),
)```