Intermittent FAIL in ` /fetch/content-length/api-and-duplicate-headers.any.worker.html`

```
OK /fetch/content-length/api-and-duplicate-headers.any.worker.html

    FAIL [expected PASS] subtest: fetch() and duplicate Content-Length/Content-Type headers

    promise_test: Unhandled rejection with value: object "TypeError: Network error occurred"
```

See https://github.com/servo/servo/runs/36295262036.