Update tests for "http.extraHeaders=<header>" to be portable back
to Apache 2.2 (the original depended on <RequireAll/> which is a
more recent feature).
* js/http-custom-headers:
submodule: ensure that -c http.extraheader is heeded
t5551: make the test for extra HTTP headers more robust
tests: adjust the configuration for Apache 2.2
848b99b14e