Re: [whatwg/fetch] CORS request-header Content-Type check should be on a combined value (#748)

So I think the main problem is in https://fetch.spec.whatwg.org/#concept-headers-append. If you do
```js
headers.append("Content-Type", "s".repeat(127));
headers.append("Content-Type", "s".repeat(127));
```
both calls will succeed for a `Headers` object with guard "`request-no-cors`".

@yutakahirano I guess we need to combine first into some temporary value, validate that, and only if that's okay do the append?

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