Fetch's request.integrity is coerced to the string "undefined" when undefined is explicitly passed

[jimsimon]

Bug Description

The following code in makeRequest coerces undefined to the string "undefined" when it's explicitly passed to the fetch options:

    // 23. If init["integrity"] exists, then set request’s integrity metadata to it.
    if ('integrity' in init) {
      request.integrity = String(init.integrity)
    }

Reproducible By

fetch('https://github.com', { integrity: undefined })

Expected Behavior

Undefined is not coerced to a string

Logs & Screenshots

Environment

Arch Linux, Node 16, server-side call (not coming from a browser)

Additional context

I ran into this while implementing a custom fetcher function for Apollo Client. Because the typings for browser/native fetch don't match unidci's fetch, I had to manually map some parameters over. It's pretty easy to workaround this by testing for a falsy value and passing an empty string if needed ({ integrity: req.integrity || ''}), but it did result in a pretty cryptic error message:

Unknown server error    Error: fetch failed

In my case, I believe my need to map stuff manually between the two implementations is actually related to #1172.

[mcollina]

Would you like to send a Pull Request to address this issue? Remember to add unit tests.

[jimsimon]

Sure, I'll whip something up!