get! for WeakKeyDict does not add elements to finalizer, so they are not removed on they keys being garbage collected

[oxinabox]

I think (and someone should check me if I am wrong)
that get! is borked for WeakKeyDict.

A key part of the functioning of WeakKeyDict, is that when an index is set,
the finalizers are hooked up, such that when that key is garbage collected,
its entry is deleted from the dictionary.

The following fails in 0.6

using Base.Test
    wkd = WeakKeyDict{Vector{Int}, Vector{Int}}()
    x = [1]
    wkd[x] = [-1]

    @test length(wkd)==1 # passes
    x = [2]
    gc()
    @test length(wkd)==0 # passes

    get!(wkd, WeakRef(x), [-2])
    @test length(wkd)==1 # passes

    x = [3]
    gc()
    @test length(wkd)==0 # fails, length(wkd)==1

    @show wkd.ht # wkd.ht = Dict(WeakRef(nothing)=>[-2]) 

Looking at the code: (https://github.com/JuliaLang/yyjulia/blob/0d7248e2ff65bd6886ba3f003bf5aeab929edab5/base/weakkeydict.jl#L109-L110)
get! on a WeakKeyDict uses to get! on a Dict, which in turn uses setindex!(::Dict...) (well _setindex2 or something),
rather than setindex!(::WeakKeyDict,...).

And thus the finalisers are never hooked in.
So the garbage collector does not remove the element from the dict.
The key does get change to WeakRef(nothing) but the value stays in the dict.
This is probably not intended

This is also way get! (unlike setindex) requires you to use WeakRef(k) for the key argument.
(similar for almost every other method. I thing all should take the specified keytype (rather than a WeakRef to it. Just like setindex! )

[laborg]

This got fixed in #38180