fix(network): do not add missing headers when looking them up

[Pavel Feldman (pavelfeldman)]

RawHeaders kept its headers in a defaultdict(dict), and get_all indexed it directly. Every lookup of a missing name stored an empty entry, so after response.header_value("x-missing") the header showed up in all_headers() as None. This affected header_value and header_values on both Request and Response.

The map is now a plain dict, filled with setdefault, and get_all reads it with .get. Lookups no longer change it. This takes the approach of #3243 a step further by dropping the defaultdict altogether.

Test: the existing test_response_headers_should_work now also looks up a missing header and checks that all_headers() doesn't gain it. It fails without the fix and passes on chromium, firefox and webkit, along with the rest of test_network.py.

Fixes: microsoft/playwright#43306