dict subclasses which override __iter__ should be treated as mappings

Here are some examples:

class test(dict):
    def __iter__(self):
        return super().__iter__()
    def keys(self):
        return ["A"]
    def __getitem__(self, key):
        return 1

A = {"A": 1}
B = test({"B": 2})

assert dict(B) == A
assert dict(**B) == A

def fn(**kw):
    return kw

assert fn(**B) == A

d = {}
d.update(B)
assert d == A

d = {}
d.update(**B)
assert d == A

This is causing an issue with order preserving dicts and collections.OrderedDict.

Reproduce the provided dict-subclass examples in IronPython, including dict(), ** unpacking, update(), and the OrderedDict case. Trace how these operations classify dict subclasses that override iter, then add focused regression coverage; done means the assertions pass and mapping order is preserved.