[xhr] Creating an ArrayBuffer can fail (#26)

When a new `ArrayBuffer` object is created this can throw a `RangeError`. We need to decide how to deal with that.

Gecko throws the exception from the `response` getter as long as allocation keeps failing. But it might be simpler to return null. That means developers won't have to expect exceptions from using a simple getter and don't have to write complicated code to keep trying to get an `ArrayBuffer` allocated in a loop.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/xhr/issues/26