3 severe issues with compound indexes

IDBKeyRange.lowerBound() and IDBKeyRange.upperBound() wont work with
compound indexes.

Doing IDBObjectStore.openCursor() without arguments also failed.

The issues were detected when testing a new unit test in Dexie that
heavily tests both compound primary keys and compound indexes.

The name of the Dexie unit test contains the string "Problem with
existing db" and is found in tests-upgrading.js in dexie github
repository.