detail::extract_params: strip leading space off name

HTTP_COOKIE are of the form
"k1=v1; k2=v2;k3=v3"

Make sure to strip leading whitespace off the name.
This fixes req.cookies.pick("k2") not finding " k2".

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>