Silence MSVC C4251 for detail::header metadata member

Author: sentientsergio

include/boost/http_proto/detail/header.hpp

@@ -44,6 +44,9 @@ struct empty
     kind param;
 };
 
+BOOST_HTTP_PROTO_MSVC_WARNING_PUSH
+BOOST_HTTP_PROTO_MSVC_DISABLE_4251_4275
+
 struct BOOST_HTTP_PROTO_DECL header
 {
     // +------------+---------+------+------------+-----------------------------+
@@ -218,6 +221,8 @@ struct BOOST_HTTP_PROTO_DECL header
         system::error_code&) noexcept;
 };
 
+BOOST_HTTP_PROTO_MSVC_WARNING_POP
+
 } // detail
 } // http_proto
 } // boost