Map<K, V> may not unify with Iterable<V>

I just stumbled into this case, and I didn't find any other open or closed issue regarding this. When a `Map` turns out to be for example `IntMap` things work as expected, however in the case of `StringMap` unification with `Iterable` fails under some circumstances.

This is not a regression and can be seen on https://try.haxe.org/#caccc

EDIT: Ok, this is not exclusive to `StringMap`, using for example `abstract X(Int)` will result in some `IntMap` that won't allow for unification with the corresponding `Iterable`.