Fix broken model.vis repr

[jjyyxx]

Closes #2488

>>> model.vis
<MjVisual
  global_: <Global
    azimuth: 90.0
    realtime: 1.0
  >
  quality: <Quality
    numquads: 4
    shadowsize: 4096
  >
  headlight: <Headlight
    active: 1
    specular: array([0.5, 0.5, 0.5], dtype=float32)
  >
  map: <Map
    actuatortendon: 2.0
    znear: 0.009999999776482582
  >
  scale: <Scale
    actuatorlength: 0.699999988079071
    slidercrank: 0.20000000298023224
  >
  rgba: <Rgba
    actuator: array([0.2 , 0.25, 0.2 , 1.  ], dtype=float32)
    slidercrank: array([0.5, 0.3, 0.8, 1. ], dtype=float32)
  >
>

[yuvaltassa]

@jjyyxx, please note slight differences between your original PR and a7a8a68:

1. Added a basic test.
2. Correct indentation.

(credit @nimrod-gileadi )