When fetching about:blank, response body should be the empty byte
sequence.
Spec: https://fetch.spec.whatwg.org/#concept-basic-fetch
Before this change, response body would be set to `ResponseBody::Empty`,
and then fetching would result in an infinite loop at step 19 in fn
`main_fetch` (methods.rs).
In function Request::fetch_main, flag local_urls_only (if set)
should allow fetching local urls only. Before this change, the flag had
the inverse behaviour.
Set response.body Asynchronously In Fetch
Following having finished making Fetch asynchronous, response.body should be set asynchronously, since it's the major goal of calling Fetch. So far, I've made the body wrapped in Arc<Mutex<>>, and I've wrapped a new thread around the part where it's set. I've also discovered that the fetch_async function makes step 8 of Main Fetch obsolete, and I've commented it appropriately.
I'm currently having a hard time with the thread for setting response.body, though. @jdm suggested I have the body set continually, block by block, but my implementation for that runs so slow that I can't finish running my fetch test suite in reasonable time. @KiChjang pointed out that a lot of the lag is due to how response.body currently stores everything inside a Vec. Changing the storage container seems to be both necessary and beyond the scope of the time I have to work on this.
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Extracted this out of #5649
This commit was created with the following commands:
```
find . -iname "*.webidl" -type f -print0 | xargs -0 sed -i '' 's/http:\(.*\)whatwg.org/https:\1whatwg.org/g'
```
```
find . -iname "*.rs" -type f -print0 | xargs -0 sed -i '' 's/http:\(.*\)whatwg.org/https:\1whatwg.org/g'
```
