Currently the `multipart/form-data` parser in
`Request.prototype.formData` and `Response.prototype.formData` decodes
non-ASCII filenames incorrectly, as if they were encoded in Latin-1
rather than UTF-8. This happens because the header section of each
`multipart/form-data` entry is decoded as Latin-1 in order to be parsed
with `Headers`, which only allows `ByteString`s, but the names and
filenames are never decoded correctly. This PR fixes this as a
post-processing step.
Note that the `multipart/form-data` parsing for this APIs in the Fetch
spec is very much underspecified, and it does not specify that names and
filenames must be decoded as UTF-8. However, it does require that the
bodies of non-`File` entries are decoded as UTF-8, and in browsers,
names and filenames always use the same encoding as the body.
Closes #19142.
This commit aligns the `fetch` API and the `Request` / `Response`
classes belonging to it to the spec. This commit enables all the
relevant `fetch` WPT tests. Spec compliance is now at around 90%.
Performance is essentially identical now (within 1% of 1.9.0).
The following used to fail in Deno despite working in the browser:
```javascript
new Request('http://localhost/', {method: 'POST', body: new URLSearchParams({hello: 'world'})}).text().then(console.log)
```
