C++ will jovially select the implicit conversion operator, even if it's
complete bogus, such as for unknown-size types or non-destructible
types. Therefore, all such conversions (which incur a copy) must
(unfortunately) be explicit so that non-copyable types continue to work.
Instead of using a HashMap<ByteString, ByteString, CaseInsensitive...>
everywhere, we now encapsulate this in a class.
Even better, the new class also allows keeping track of multiple headers
with the same name! This will make it possible for HTTP responses to
actually retain all their headers on the perilous journey from
RequestServer to LibWeb.
(cherry picked from commit e636851481eabdf00953573a5eb459ee52feeacc)
Updated various SerenityOS components to make it build.
Fetch: Make sure we iterate over HeaderMap's headers()
This fixes a build failure when built with CMake option
'-DENABLE_ALL_THE_DEBUG_MACROS=ON'.
(cherry picked from commit c51d01bea712d75f9b2cd700be942935044e49b4)
