has(key) returns true after last item has been deleted using delete(key, value)

[Kered13]

The bug is demonstrated in the example REPL session below:

> m = new Multimap()
Multimap { _map: [Function: Map], _: Map(0) {} }
> m.has('a')
false
> m.set('a', 1)
Multimap { _map: [Function: Map], _: Map(1) { 'a' => [ 1 ] } }
> m.has('a')
true
> m.delete('a', 1)
true
> m.has('a')
true
> m.get('a')
[]

In the second to last line, false is expected as all values for a have been deleted. In the last line, undefined is expected as no values exist for a.