Improve handling of list valued headers
Improve handling of list valued headers

Use a hashtable to check if a header key is list valued rather than a
map

dsheets/ocaml-cohttp,fdopen/ocaml-cohttp,mor1/ocaml-cohttp,inhabitedtype/ocaml-cohttp,mor1/ocaml-cohttp,edwintorok/ocaml-cohttp,fdopen/ocaml-cohttp,marklrh/ocaml-cohttp,edwintorok/ocaml-cohttp,Drup/ocaml-cohttp,Drup/ocaml-cohttp,marklrh/ocaml-cohttp,fdopen/ocaml-cohttp,dsheets/ocaml-cohttp,dsheets/ocaml-cohttp,Drup/ocaml-cohttp,inhabitedtype/ocaml-cohttp,edwintorok/ocaml-cohttp,inhabitedtype/ocaml-cohttp,marklrh/ocaml-cohttp,mor1/ocaml-cohttp