[w3c/IndexedDB] Array proxies as keys? (#309)

I noticed that this throws in both Chrome and Firefox:

```js
const k = [1,2,3];
const p = new Proxy(k, {});
indexedDB.cmp(k, p);
```
In Chrome, at least, the proxy fails an internal "is this an array?" check. 

In the spec [convert a value to a key](https://w3c.github.io/IndexedDB/#convert-value-to-key) calls ECMAScript's [IsArray](https://tc39.es/ecma262/#sec-isarray) which would seem to return true for array proxies. 

At the very least, we don't have tests. 

Should we update the spec to exclude proxies, e.g. replacing _"if IsArray(input)"_ with _"if input is an Array exotic object"_?

Alternately, should array proxies be supported? What are the consequences of allowing these? Would the value→key conversion algorithm potentially blow up in more places?

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