More reliable test_post_with_headers

[wimglenn]

This will make the test fail if the mock hasn't matched (in the original PR it failed for other reason, earlier, not because of a mock mismatch).

By using https://httpbin.org/foo instead of https://example.com, the resp.status will be 404 instead of 200 if the mock didn't apply successfully. That's a less confusing failure mode.

[sarayourfriend]

Thanks @wimglenn 🚀