.insert_all() fails if subsequent chunks contain additional columns

[simonw]

Reported by @nieuwenhoven in #225 along with a proposed fix.

[simonw]

I managed to replicate this in a test:

def test_insert_all_with_extra_columns_in_later_chunks(fresh_db):
    chunk = [
        {"record": "Record 1"},
        {"record": "Record 2"},
        {"record": "Record 3"},
        {"record": "Record 4", "extra": 1},
    ]
    fresh_db["t"].insert_all(chunk, batch_size=2, alter=True)
    assert list(fresh_db["t"].rows) == [
        {"record": "Record 1", "extra": None},
        {"record": "Record 2", "extra": None},
        {"record": "Record 3", "extra": None},
        {"record": "Record 4", "extra": 1},
    ]