items() values drop the key fields when there is no getitem_projection

[thorwhalen]

The gap

MongoCollectionReader.iter_items builds each item's value by popping the key
fields out of the document:

for doc in cursor:
    key = {k: doc.pop(k) for k in self.key_fields}
    yield (key, doc)

That is right when getitem_projection excludes the key fields (the usual
configuration): __getitem__ wouldn't return them either.

It is wrong when getitem_projection is None, which means "return the whole
document". Then store[k] includes _id while items()' value does not, so

list(store.items()) != [(k, store[k]) for k in store]

— a Mapping-contract violation, in the default configuration of every
MongoCollection*Persister.

values() is unaffected: iter_values projects with getitem_projection and
pops nothing.

Why it wasn't fixed alongside #7

The correct behaviour (strip only the key fields the getitem projection would
not have returned; never mutate the doc when it is None) changes results that
mongodol/tests/int_tests/base_int_test.py::test_store_with_mappers explicitly
asserts:

assert list(store.items()) == [
    (key_output_mapper({ID: n[ID]}), value_output_mapper({k: v for k, v in n.items() if k != ID}))
    for n in nums_and_lans
]

so it needs a deliberate decision about which of the two is the contract before
the test is rewritten.

Where it is pinned

mongodol/tests/views_test.py::test_items_values_equal_getitem_values_when_no_getitem_projection
is a strict=True xfail, so it will flip the suite red the moment the behaviour
is fixed — and points here.

Surfaced while fixing #7.