integration tests with specific header expectations

[eric846]

Currently some integration tests have relaxed requirements about things like byte counts (e.g. >=500). This means when headers produced by Nighthawk or the test server change unexpectedly, the tests continue to pass even though the byte counts have changed.

We may want to create additioanl integration tests that enforce specific assumptions about headers.

[eric846]

As of #641, some tests had strict equality counts, but for example test_http_h2 had some relaxed checks. We probably want all similar tests to either be uniformly strict or lax.