The Fetch spec defines HTTP whitespace as tab, LF, CR, and space.
Previously, trim_whitespace was also stripping vertical tab (U+000B)
and form feed (U+000C), which are not HTTP whitespace characters.
Switch to HTTP::normalize_header_value which matches the fetch
definition.
Fixes 4 subtests for WPT test:
https://wpt.live/cors/origin.htm
