Non-http fetch doesn't set response URL

[Manishearth]

XHR needs to know the response URL (https://fetch.spec.whatwg.org/#concept-response-url). However, datauri/about/etc fetches don't set it.

We should probably set the response URL in basic fetch itself.

[annevk]

This regressed in ed37f5e. 😟

[annevk]

Maybe a way to prevent further mistakes is to figure out a way to make this field immutable and set at the time we construct a response rather than this rather weird approach.

[annevk]

I think I should just move this back to main fetch. There's no real reason to do it locally.