[xhr] Resture the filename argument to FormData.append(name, blob) (#25)

Context: https://bugzilla.mozilla.org/show_bug.cgi?id=1162658

I would propose this IDL:
```WebIDL
void append(USVString name, Blob value, optional USVString filename);
void append(USVString name, USVString value);
```

(This is what Blink does.)

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/xhr/issues/25