[IndexedDB] Allow multiEntry + Array keyPath (#36)

Imported from https://www.w3.org/Bugs/Public/show_bug.cgi?id=21836

Currently, the spec for [IDBObjectStore.createIndex](https://w3c.github.io/IndexedDB/#dom-idbobjectstore-createindex) has:

> If keyPath is a sequence and multiEntry is set, throw an InvalidAccessError exception.

This disallows some interesting use cases, although the expected behavior in more complex cases is unclear. Per @sicking:

Consider a multiEntry index with keyPath ["a", "b"]

It's pretty intuitive that storing a value like:
```
{
  id: 1,
  a: "apple",
  b: "orange"
}
```
would add the following entries to the index:

"apple" -> 1
"orange" -> 1

But what would storing the following value result in?
```
{
  id: 1,
  a: ["apple", "x"],
  b: ["orange", "y"]
}
```
Would this result in the following entries

A)
["apple", "x"] -> 1
["orange", "y"] -> 1

or

B)
"apple" -> 1
"x" -> 1
"orange" -> 1
"y" -> 1

or

C)
["apple", "orange"] -> 1
["x", "y"] -> 1


---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/IndexedDB/issues/36