Defaulting None for optional fields

[earlbread]

This fixes #70.

[dahlia]

@earlbread Generally it looks good to me. 👍 We have only few things to fix.

[dahlia]

We should test union tags as well e.g.:

union union-test = tag-a
                 | tag-b (text a, text? b, int64 c, int64 d?);

[dahlia]

We should test union tags as well e.g.:

def test_union_tags_optional_initializer():
    t = TagB(a=u'test', c=123)
    assert t.a == u'test'
    assert t.b is None
    assert t.c == 123
    assert t.d is None

[dahlia]

Since the function name already starts with test_ we need no _test suffix here.

[earlbread]

@dahlia I fixed things you mentioned. Thank you!

[dahlia]

Everything looks good to me. 👍