fix: Missing header detection for the non-strict case

[woodyza]

Hi!

When #651 was implemented the header matching rule was subtly broken, making it no longer detect when the specified headers were missing entirely (vs present, but with the wrong value) unless you used strict matching.

As far as I understand that is not the intention, and you should be able to use the header matcher to detect missing headers without necessarily caring about all of the other headers in the request. It's a small change to enable that, and it preserves the strict match and regex match logic.

I've added a test case that fails without the current change, and the rest of the cases still pass with it in place.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (89d7f37) 99.93% compared to head (dd7b873) 99.93%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #702 +/- ## ======================================= Coverage 99.93% 99.93% ======================================= Files 9 9 Lines 3054 3064 +10 ======================================= + Hits 3052 3062 +10 Misses 2 2 ```

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[markstory]

Thank you 🎉

[woodyza]

Thanks @markstory for the merge 🙏 Any idea when 0.25.0 might come out?

[markstory]

@woodyza I started a release build just now.

[woodyza]

Awesome, thanks!