Handle unpersisted primary key values

[smudge]

Two changes here:

1. I renamed Object#primary_key_value to Object#bullet_primary_key_value to make it clear that this core extension is specific to bullet and to avoid potential naming conflicts. (This feels at least to me like a semi-private method that shouldn't be used by external code...)
2. I added a test case and made code modifications to handle the case when a primary key value has been manually set on an unpersisted record. One might commonly do this, for instance, in a test context (to avoid making unnecessary database round-trips).

Here's a very contrived example:

# Totally make-believe test case:

post = user.posts.build(id: 123)
comment = post.comments.build(text: 'hey, cool post')
email = WeeklySummaryEmail.new(user: user)

expect(email.subject).to eq "1 new comment on post 123!"
# `email.subject` has code to traverse user->posts->comments

Previously, bullet would prevent this test case from passing, even though the model does not get persisted at all, simply because the id value is present. This change allows bullet to additionally rely on persisted? (if it is defined) to decide whether a record might be subject to N+1 issues, etc.