Returns the value for a case-insensitive matching
header, or "".
boost::string_ref const& name) const;
If more than one field with the specified name exists, the first field
defined by insertion order is returned.
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)