GLTF 302 Redirects not taken into account when loading assets

### Description

Requesting a gltf file that resolves via a 30X redirect doesn't rewrite relative asset paths to that new location, resulting in 404s.

### Example:

1.gltf has image 'image.png' stored in same directory

**Request**: api.example.com/models/gltfs/1
**Response**: _302_ cdn.example.com/gltfs/1.gltf

**Model Viewer**: _404_ api.example.com/models/gltfs/image.png

Note that Scene Viewer on Android handles the redirect correctly.

#### Live Demo

https://glitch.com/edit/#!/join/1e825f92-f5fc-4984-909c-58d23b090db6

### Version

- model-viewer: v1.1.0

### Browser Affected

- [x] Chrome
- [ ] Edge
- [ ] Firefox
- [ ] IE
- [ ] Safari

### OS

- [ ] Android
- [ ] iOS
- [ ] Linux
- [x] MacOS
- [ ] Windows