be_delivered expectation fails when it shouldn't

Nobody has claimed this yet.

I have a spec that looks like this:

```crystal
NewPostEmail.new(friend, me, post).should be_delivered
NewPostEmail.new(friend, me, later_post).should_not be_delivered
```
because I'm sending bulk emails. In this case, I want to make sure the first one is sent because `post` is valid for sending at this time, but `later_post` is not, and should not be sent out. When the spec runs, it says both should be delivered, but I think it's a false positive because it sees the same `to` and same `subject` lines and probably whatever else to determine that these are the same email even though they're not.

Start with the shown Crystal spec and trace how the be_delivered expectation identifies an email, especially which message fields it compares. Reproduce the case with two emails sharing a recipient and subject but using different posts. Done means the expectation distinguishes the two messages and the first passes while the second fails as expected.

Written by the indexing model from the issue text.