Add support for optional default headers

[camflint]

This change adds support for default headers.

import wptserve

config = {
    "host": "localhost",
    "domains": {
        "": "localhost"
    },
    "ports": {
        "http": [8000]
    },
    # new!
    "default_headers": [
        ["Cache-Control", "no-cache, no-store"]
    ]
}

httpd = wptserve.WebTestHttpd(config=config)
httpd.start(block=True)

Default headers are useful in setting the default cache policy for wptserve when serving test files for Internet Explorer and Edge in dev and test environments, so that test files are not inadvertently cached between runs. This prevents "F5-refresh" headaches when a developer makes changes to the tests locally, and prevents leftover state on shared machines (which may need to run multiple versions of the tests) from affecting subsequent test runs.

This feature is meant to be complementary to specifying per-resource headers in .header files, header pipes, Python scripts, and so forth. Any of these explicit mechanisms will override the default headers.

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This change is

[gsnedders]

The code LGTM, and I'm in favour of the feature. I'll let others decide how much in favour of the feature they are!

[jgraham]

This makes me nervous. A test that is expecting certain headers might unexpectedly fail when different headers are provided in the defaults. There's also more than one way to inadvertently store information locally so tests will fail if not run from a clean state (e.g. in local storage).

[camflint]

@jgraham I see where you're coming from, in that it makes sense in principle for tests to take care of their own cleanup before/after runs to minimize environmental dependencies. In the example given it's a bit of a chicken-and-egg problem, because if the browser is secretly caching test.html and I change test.html to clean up after itself and refresh the page, my new test bits aren't going loaded, and my cleanup code wouldn't run. I see the danger in using default_headers too liberally, but I do think it's useful and warranted in this scenario, and maybe some other edge cases.