Choose getIn based off Iterable.isIterable() (#2423)

Currently this breaks when using `Record`s or other keyed Immutable objects that aren't `Map`s or `List`s. I've tested this in the context of an app I'm working on and these changes work in my case, but haven't verified with any tests. All `Iterable`s have `getIn` so it's a very low risk change.

src/structure/immutable/index.js

@@ -12,7 +12,7 @@ const structure = {
   empty: Map(),
   emptyList,
   getIn: (state, field) =>
-    Map.isMap(state) || List.isList(state) ? state.getIn(toPath(field)) : plainGetIn(state, field),
+    Iterable.isIterable(state) ? state.getIn(toPath(field)) : plainGetIn(state, field),
   setIn,
   deepEqual,
   deleteIn: (state, field) => state.deleteIn(toPath(field)),